Bryan's Presto 92B project - Buying one in AS-IS condition ;

[EpicenterBryan]

Hi guys,
Thought I would start a new thread about a Presto 92B Amp that I purchased several months ago In Tacoma, WA in AS-IS condition. It was the one listed back in May 2018 in a post in the classifieds on LatheTrolls... https://www.lathetrolls.com/viewtopic.php?f=4&t=7821

Here is a back shot showing it was missing tubes, the transformer cover, and the high voltage wires for the 807 tubes were cut off. The seller mentioned that one guy who was interested in this amp had a spare back cover but he never gave me his name or contact info. If you have one, drop me a line before I have one made! I have one from my 92A that can be used as a pattern if I need...

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The repair and testing of this amp is a bit out of sequence, but here are some early photos for tonight. Details to follow...

New 40KV wire I found on E-bay - WAY over rated, but multi strand and silver coated so it soldered well...

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Also shown are new ceramic / gold plated connectors for the anodes of the 807's.

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That's all I have time for tonight, but this multi month project will be interesting to many people I think. The down side is it will take me time to catch up posting all I've learned and what has been done. As of tonight, I'm still waiting for 2 capacitors to arrive so perhaps I can catch up by the time the final capacitors get here!

For tonight - Enjoy.

B

[Kris D]

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[EpicenterBryan]

Kris D rocks!
Drop me a line with my cost!

Bryan

[piaptk]

Of course Kris has (at least) one of those.

[EpicenterBryan]

Let me voice one major bitch about how the forum works. I'm on the west coast. The site time seems to be related to the east coast and if I'm working on a post out here on the west coast, many times the entire post is lost while I'm working on it because of a three hour time difference and some "clock switch over" or something. I just don't know, but this is just one of many times I've had to recreate posts and the site doesn't save any "versions" of them. Tonight, my entire post was lost and I'm starting over again.

NOT HAPPY STEVE!

Fortunately, I saved some preliminary text and will try to re-construct what I can in a shorter post tonight.
Steve, please fix this!

Here goes with what I recovered:
----------------------------------------------------------------------------------------
Starting to add stuff as time permits - so here is tonight's installment:

The first task was to check out the power supply.
Below is a chunk of the schematic with the HV P/S section for the first test. The three tubes in red (the only ones that were included) were removed. Power was applied to the input (green) and all voltages in yellow were tested. The transformer was good! One HVAC measurement is shown below.

power supply-1.jpg

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While the three included tubes were out, I tested them on my Go-No-Go Drug Store Tube tester, and two tested OK, and one not. Then I discovered this...

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One of the 5R4 rectifiers actually had a broken and totally missing orientation pin. Once I realized that and plugged it in correctly into my tube tester it checked out as functional, but I wonder if it had been installed in the chassis incorrectly and may have contributed to the Amp being shelved years ago... I ended up tossing that tube but saved the other one. Eventually I used a pair of military NOS tubes I had as spares for my 92A. Anyway, the third tube (OD3) tested fine.

IMG_6160.JPG

Can some one please explain what causes a post to get ditched while you are working on it? Especially when you have been working on it for some time?

B

[soeffingodly]

Let me voice one major bitch about how the forum works. I'm on the west coast. The site time seems to be related to the east coast and if I'm working on a post out here on the west coast, many times the entire post is lost while I'm working on it because of a three hour time difference and some "clock switch over" or something. I just don't know, but this is just one of many times I've had to recreate posts and the site doesn't save any "versions" of them. Tonight, my entire post was lost and I'm starting over again.

Can some one please explain what causes a post to get ditched while you are working on it? Especially when you have been working on it for some time?

B

Hi Bryan, I can't with 100% certainty say what the cause is but I know I have seen similar behavior on my work client area site since it does some "auto saving" and whatnot. Did the text just outright disappear? Or did the page refresh and the text went buh-bye at that point? May also be a caching thing and the web server cache gets recycled every X period? Have you tried using the Save Draft feature as you work? That is what I had to resort to when dealing with my work client area website so I didn't lose content I was writing. But Steve may also be able to tweak some settings in the back end. Or at the web hosting level. I am not entirely sure. It has been many, many years since I administered a BB forum.

PS fantastic work/post! I love cramming this stuff in my brain for possible use later. =)

[EpicenterBryan]

Did the text just outright disappear? Or did the page refresh and the text went buh-bye at that point?

It just went away when I hit the preview button. I use preview to make sure I put the photos in the right place and to make sure text is formatted the way I intended, so I use it several times per post.

I haven't used the save draft button in a long time. I don't remember details but I recall running into a similar problem with that years ago.

Sorry for my bitch session last night.

B

[soeffingodly]

Did the text just outright disappear? Or did the page refresh and the text went buh-bye at that point?

It just went away when I hit the preview button. I use preview to make sure I put the photos in the right place and to make sure text is formatted the way I intended, so I use it several times per post.

I haven't used the save draft button in a long time. I don't remember details but I recall running into a similar problem with that years ago.

Sorry for my bitch session last night.

B

No worries! I've had that happen so I can relate. I bet it is a caching/timeout type of thing. I guess in the meantime just keep a copy of what you typed up before clicking preview or submit...

[EpicenterBryan]

Before we get started on the next segment, let me throw out a few precautionary notes:

1). Tube amplifies have High Voltages inside the chassis.
2). These voltages can be stored via capacitors long after the unit is unplugged from power.
3). Discharge ALL capacitors before probing around!
4). Follow the "One Hand" rule! Keep one hand behind your back when you are probing around. If you accidentally make a circuit with both hands while HV is present, you could wake up dead (or not).

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Having said that:
Here is a resistor with clip leads I used to discharge all the capacitors (one at a time) before I put my hands in there. I gave it 10 seconds for each discharge. You could figure it out using the RC time constant for this resistor and what C you were discharging - but 10 seconds is more than safe...

IMG_6293.JPG

The next step was to check out the critical capacitors before applying power, and with the tubes removed.
C14 could easily be disconnected. C15 (A/B, C, and D) could not be easily disconnected. All were tested for shorts with an Ohm meter, and all were tested with a capacitance meter. Although ideally all C15 caps should have been out of circuit, at least they did not test as shorts with an Ohm meter. Oh, and the F2 Fuse was fine. Again, Red means removed, Yellow is what was checked.

power supply-2.jpg

The next step was to install both 5R4 tubes, with the OD3 removed to check the 460V DC with C14 connected.

power supply-3.jpg

For this test, I used a Vari-AC to bring the line voltage up slowly until it was at correct voltage while monitoring the DC across C14.

Mark will appreciate this Vari-AC! I bought this (and one for MarkRob too) at an estate sale a few years ago for $7.50 each on the last day! It cost 3 times that to mail one to Mark...

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As shown, the 460VDC was good to go...

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That's where I'll stop tonight so I don't loose anything! Much more to come...

B

[EpicenterBryan]

Some more back fill for tonight:

I installed the voltage regulator tube, and again brought the amp up with the Vari-AC while measuring the voltage across C15 A/B. It looked good. Although I don't have notes about checking the other two HV points I probably did that too but I'm not sure. Those voltages are not shown on the schematic but can be easily calculated with the resistors shown...

power supply-4.jpg

At this point I convinced myself to install the tubes and do a quick test of the tube currents using the front panel switch. What some people may not know, is that this switch can be used to make checks on relative tube current. It's kind of like a built in tester so you have an idea if a tube is starting to go. The readings should be within the 10-0-10 range on the meter.

The letters A-G relate to the tube designations on the schematic and the back of the chassis. An example: "A" is the first pair of 807's and "B" is the other pair of 807's. If switching between those two positions shows different numbers (within reason) there is a balance control that can be adjusted and I did that since I know all four 807's had about the same readings on my tube tester. But, that was only after I fixed another problem...

I discovered one of the 807 tube filaments were not lighting up. Moving the tube out slightly made them come on. Pushing in slightly and the tube would shut off. I ended up using a dental pick to hook the contacts and adjust their shape so they made better contact with the tube pins and problem solved! Wow, much easier than replacing a socket! But remember, you always need to discharge all the capacitors before you go poking around in a chassis or a socket! And yes, I even put heat shrink on the metal handle just in-case.

Switch and meter.jpg

Then I was ready to crank some tunes. First off, this amp has what the manual says was an optional 15K ohm input transformer rather than the one that could be jumped for 50, 200 or 500 ohm. That's pretty cool! I ended up using an iPhone as the music source.

As you might know, this amp has outputs for 6, 15, 250 and 500 ohms. So I used the 6 ohm output and connected to a 6 ohm KEF Reference RDM2. Yep, that speaker is actually 6 ohm.

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So how did it sound?

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AWESOME!


But, there was a slight 60 cycle hum that was constant even with the input pot down all the way. So, after talking with MarkRob, he suggested pulling the tubes except the ones for the power supply, and the four 807's and yes, the hum was still there. So, the issue was with the filtering in the power supply section (Capacitors). Pretty typical for an amp of this age. I made some AC measurements on the DC points in the supply, and yes, there was AC ripple at those points. I don't have my notes on those readings anymore but MarkRob convinced me it was time to just re-cap the whole thing. That's where I'll leave off for tonight.

And this is where it gets interesting. Replacing all the Capacitors!

But first, here are a few more photos for back fill.
First, here is my Tube tester now with updated dimable LED lighting.

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And when I bought it, I also bought a full set of tubes as spare for my 92A. These are the ones I'm using for the 92B. Also shown are some tubes I also picked up as spares for another one of my amps. And, the guy threw in some extra 807's in mismatched brands, and a spare meter for free!

spare tubes.jpg

This tester did not have a socket to be able to test 807's directly, but on E-bay I was able to purchase an adapter so I could test 807's like they were 6L6's!

807 adapter.jpg

That's it for now.
For those who are in the path of Hurricane Florence tonight - be safe.

B

[EpicenterBryan]

Tonight, I'll start on some of the capacitor replacement back fill...
First, I ordered some phenolic terminal strips in several versions. Mostly 3 terminal (two independent, with a ground terminal). Those could be cut down so I could make 2 terminal ones in either right or left orientation. I also ordered some 4 and 5 terminal strips. I also bought two versions of Terminal Turret boards that I thought I may use for mounting discrete electrolytic's in place of two of the cans. More about that later. Some times, I like to get some stuff on order while I'm trying to figure out how I'm going to do something. None of this stuff was expensive.

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The idea about the Turret boards was to provide an easy place to mount capacitors near their original location but to eliminate two of the "twist lock" can type capacitors that were mounted internally. These two in particular:
Notice a new axial cap is shown as already being replaced - oops. Wait for it later...

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Here are two ideas about where I would mount a section of a Turret Board. Either location would have worked using existing holes with the addition of some 3D printed adapter / spacers...

This location near the transformer...

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Or this position under their original location...

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Two of the other twist lock can capacitors were mounter outside the chassis with terminals passing into the chassis.

Let me jump ahead. These are (most) of the capacitors I've replaced as of today.

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And this is where I have to end the post tonight. Will do anouther update tomorrow.

B

[EpicenterBryan]

Oops. Looks like there was an error in the last post.
Here is the correct photo showing a possible location for the turret board near the transformer:

IMG_6295.JPG

With those two photos you are starting to see a few things that I want to hold off discussing until later. So let me continue with some more back fill.

While looking over the wiring and deciding which way I wanted to go with two of the can capacitors, I discover a ground wire with significant distortion in the insulation. It looks like this wire carried too much current at some point. I replaced it with a larger gauge wire.

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There were three externally mounted caps like shown below. Two were multi section twist lock cans. I decided to move all three to the inside.

External caps.jpg

What that allowed me to do was use the existing holes to mount a standard IEC power input connector!

And, I was able to install a Neutrik Combo series connector so I now have an XLR and TRS Input!
No drilling required. I just 3D printed some adapters. For the two holes left by moving the bath tube capacitor, I installed some rubber hole plugs.

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Going back to the two twist lock cans that I was thinking about using a turret board for, I decided to see if I could cut one open to save the pins, and the twist lock ring. The idea was then to use those parts to hold new capacitors. A pipe cutter worked to get to the parts.

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Eventually, I decided it might be easier to just make a little bolt on circuit board with the new caps installed and lugs on the other side to solder the wires to.

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And then I discovered a new path!

First off, the reason so much effort went into what to do about the twist lock cans is this: Although you can find some new ones out there, the exact combination of number of capacitors in the can, the capacitance of each and working voltage of each makes it nearly impossible unless you are working on something fairly modern or common - like a fender amp. You can literately spend days going through every vendors listings searching for something as I did. A few times I was close, but the can wasn't a twist lock, so I would have to drill holes to install a clamp to hold the capacitor in the same position.

I found a company who makes custom twist locks!
And there is no minimum number required. They will make just one if you need.

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The awesome thing about these guys is you give them some details like this, and they send back a quote.
1) 3 twist tabs, or 4 twist tabs
2) Desired height (default is 2.5")
3) Number of sections, and capacitance and voltage for each section
4) If the outside of the can is insulated or not
5) If a mounting wafer is needed at time of order
6) Polarity of can common

What did my two custom cans cost?
$73.70 including shipping. That's right! Including shipping.
Turnaround time was crazy. I had them in my hand in 4 days.

They are Hayseed Hamfest LLC.
Their website is https://hayseedhamfest.com/

I have one more capacitor to install that came this week and then I need to test the amp. After that I'll post final photos of the guts.

Wish me luck.

B

[Radardoug]

You have an Ampex MM1000? Woohoo!

[EpicenterBryan]

You have an Ampex MM1000? Woohoo!

I do! Pretty cool that you caught that. It's briefly mentioned on this thread page: Scroll down 3/4 of the page:
https://www.lathetrolls.com/viewtopic.php?f=15&t=5462

And I did a video on cleaning the switches here on you tube:

https://www.youtube.com/watch?v=hcIu-piK_0c

B

[EpicenterBryan]

More stuff for tonight:

One thing I wanted to mention was that I accidentally touched the insulation on one of the wires with my soldering iron and burned a spot. This was not one of the HV wires, but it was hard enough to get back in there after the fact to try to replace it, and I didn't want to just put electrical tape on it. As we all know, that glue breaks down with time.

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I found a self bonding silicon type wrap a while back and you can find in the electrical department at Home Depot for example. It's rated at 400V per mil thickness. You wrap it and tension it as needed and once it touches itself it starts to bond. Within a few minutes, you can't unwrap it and have to cut it off. Crazy stuff...

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I cut a thin strip and covered the wound with two wraps and good as new... That other black spot is not a burn spot, it's old electrical tape scum. I probably should have cleaned that so no one could get confused.

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Before I get to the test results...
let me show you the before photo:

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And the after photo:

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Pretty much everything is where it was originally, with the exception of the externally mounted caps in the lower left that got moved inside so an AC input jack and an audio input jack could be added. And you will also notice I marked all the capacitors with their designations from the schematic at the same time making sure all values could be read facing the viewer. I can't tell you how pissed off I was when the values of those two cans couldn't be read from the front and I had to use an endoscope to read them around the back side. What were thy thinking?

I'm never thrilled about having to put something over the top of something else (like C5 over the top of C4AB and C4CD) but sometimes it is what it is.

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See how nice it is to read caps when oriented like this?

That's it for this post. More stuff to come.

B

[EpicenterBryan]

More details to come.
But here is a quick sound check video...
Audio is cut off several times so you can hear what the background 60hz is.
Also shown are the tube currents for all the tubes and how close they are in the 10 - 0 - 10 range as specified in the manual.
Enjoy!

B

[EpicenterBryan]

I had to delete the video in the previous post because of the music.
For those who got to hear it, that sounded damn nice even filmed with an iPhone.

More stuff for tonight:

I don't think I mentioned but there is a balance control for the 807 "A" pair, and 807 "B" pair. That pot was adjusted to make the meter in the "A" position equal the meter reading in the "B" position as shown:

807 balance.jpg

I decided to see how well I could null out any 60hz hum. The procedure is to remove the 6SN7 tube, and place a 500 ohm load on the 500 ohm output taps on the transformer. Then the measured AC across the load is monitored while adjusting the hum balance pot (R20). The measurement should be less than -40dbm which works out to 9mv AC. The best I could do was adjust to 17mv. I decided to swap out both 6SJ7 tubes (which tested at 60 and 68 on my tube tester) with another pair (which tested at 76 and 76) and was able to get the hum trimmed down to 14mv.

I noticed that adjustment of the pot was really touchy and I've decided to replace it with a 10 turn so I can see if it's possible to get it closer to what they discussed in the manual. That should be here in a week.

I decided to do a quick Frequency check for the Flat, 78 and NAB lateral eq settings. I'm going to need to build an attenuator to make the measurement at normal power settings, but for low volume the charts look great! These were measured on the 500 ohm load.

Here is what the Flat setting looked like:

flat.JPG

Here is the 78 setting:

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And here is the NAB Lateral setting:

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And here is what the manual shows for all three switch settings.
It also shows response for NAB Vert which is not on this amp.

92A response.jpg

Remember, the lower part of the response is actually built into the 1D head as shown below, with the combination of an RC network in series with the head:

1D head response.jpg

CONTINUED IN NEXT POST:

[EpicenterBryan]

The next step was to calibrate the meter in the "output" switch position.
Again, I had a 500 ohm load on the 500 ohm taps. My load has a switch for selecting the impedance, a range selector and has a meter built in.

Per the manual, Calibration of the meter is done with an input source of 1khz. The measurement across the load should be 24.5 VAC (+30.8dbm) which they say is the correct level for a 500 ohm 1D.

I used 2 meters. One to measure the voltage directly like this:

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And my crazy Howel-Ronset TS-585DU Output meter box...
The way this works is a bit odd: If reading dbm, you use the numbers in red (reading is slightly less than 11 as shown), and you add the number in the scaling knob setting (+20) to that reading . So the final number is 11+20 = 31dbm.

load reading.jpg

With that said - the meter is calibrated!

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B