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EpicenterBryan
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Schematic / advice needed: Presto K11

Post: # 36624Unread post EpicenterBryan
Tue Aug 11, 2015 10:22 pm

Hi guys,
I'm in need of a schematic for a Presto K11.
This is kind of an odd one and believe it or not, Alan Graves doesn't have a schematic for it. Alan's site notes it was released in 1956.

Below is a picture of mine.
presto1.gif
The amp produces a distorted waveform on the negative portion when the "tone" knob is rotated toward "treble", and perhaps also when the output is increased (it's been a while since I had a scope hooked up to the output). The Positive waveform is fine. I have a tube tester and the tubes test OK. There is a pot connected to wires in the AC transformer / 5Y3 tube / first can filter caps so I assume that may have something to do with high voltage positive and negative balance?

This amp uses the following tubes:
5Y3
6AU6
6V6
12AX7

I figured I would go ahead and recap the whole amp to start. It has two milti-value electrolytic cans (4 values in each), and also a half dozen obviously paper 600V caps. But there are a few mystery parts. Two look like they could be either mica caps (most people say don't replace them as they are reliable) but at least one looks like it may be a paper cap in the same style package. The color coded dots are not vibrant so I'm not 100% sure what the value or working voltage would be. These two caps are connected to the bass / treble pot so I'm fairly sure they are involved with tone.
IMG_3899.JPG
IMG_3900.JPG
Then there are these mystery parts. The two parts in series that are green have black bodies. I have read that some paper caps were also in these black bodies. No clue what the value would be. And then there is at least one mystery part inside a semi clear tube. Not sure what that is without cutting it open. There might be two items in there - just not sure. I can't think of anything that might have a body shape like that unless it's an obsolete diode or something.
IMG_3901.JPG
I have zero experience with tube gear, so any advice would be appreciated. And any form of a schematic would be oh so awesome!

Bryan
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markrob
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Location: Philadelphia Area

Re: Schematic / advice needed: Presto K11

Post: # 36635Unread post markrob
Wed Aug 12, 2015 6:18 am

Hi,

The two rectangular caps are mica's. I've never seen a cap of this type be anything else, but you never know. I'd leave them alone.

The green mystery caps might be low voltage electrolytics. These were typically used as cathode bypass caps (same function as an emitter bypass in a solid state circuit). Try unsoldering one leg and test with a DMM that has a capacitance range. If they read in the uf range, you have ID'ed them. Also test as a diode (if you DMM has that feature). By 1956, these were getting to be more common. They could be part of a crude limiter to prevent overdrive to the head.

Not sure what the device is with the sleeving. Again, unsolder one leg and slip off the tubing and shoot a photo if you can. Try testing as a diode.

Some troubleshooting tips.

Download the RCA tube manual. Its a great reference for these types of equipment. Besides the pinouts and specs for the tubes, you will also see typical application circuits for amplifiers. Look for one with a single 6V6 and you will get a very good idea of what to expect.

Check the B+ voltages at each terminal of the mutlisection caps. If you follow the circuit flow from the cathodes of 5Y3 rectifier tube, you will see that these terminate to the first filter cap. This will be the highest B+ point. Look for a voltage in the 250 -350 Vdc range here (the RCA tube manual will show typical class A voltages for a 6V6). From there, you'll see each section of the filter cap is connected to the previous by a dropping resistor. Each of these points feed a lower voltage B+ to earlier stages. Make sure you have voltage at each lug.

If you have a scope that allows you to AC couple its input and you have a 10X probe, you should be able to feed a signal into the input and follow the audio from input preamp to head. If you download an RCA tube manual, you can check the pinout of the 3 tubes (12AX7 dual triode, 6AU6 pentode, and 6V6 power output). You should be able to trace which tube is first in the audio chain by following the input jack to the grid of the first tube. Once you locate this, just move from grid (input of the stage) to plate (output) with your scope probe. Look for the first stage that shows the trouble and concentrate there. The 6AU6 is a bit more tricky since it has more than one grid. You should be able to ID which one is the signal grid by tracing the wiring (it will be connected to the previous stage plate via a paper coupling cap). The RCA tube manual should also give a good clue. These circuits are pretty simple, so you should be able to troubleshoot armed with a scope DMM, signal source and tube manual.


Hope that helps.

Mark

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EpicenterBryan
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Re: Schematic / advice needed: Presto K11

Post: # 36716Unread post EpicenterBryan
Fri Aug 14, 2015 6:51 pm

markrob wrote: ...Hope that helps.
It sure does Mark! Thanks for the inputs.
As I've found, there are many examples of single 6V6 amps. Your whole discussion of the HV section around the 5Y3 makes sense now that I have seen several schematics.

I have most parts on order and will go through the procedure you outlined. I'll take a look at the green and clear tube parts this weekend.

Thanks again for the help!

Bryan

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EpicenterBryan
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Re: Schematic / advice needed: Presto K11

Post: # 36887Unread post EpicenterBryan
Sun Aug 23, 2015 2:22 pm

Well, I got some stuff done. All 5 of the paper caps have been replaced with film caps (the orange ones). And all 8 segments from the 2 can capacitors have been replaced. The electrolytics are hot glued to perf boards and zip tied as well.
IMG_3933.JPG
The black discs cover the mounting holes where the Capacitor cans were.
The head output is connected to an 8 ohm 2 way speaker. The head on this presto is 8 ohm. An iPhone is being used as the audio source.
IMG_3935.JPG
All of the paper caps tested OK out of circuit which surprised me. Only 1 segment of 1 can tested really bad. A 20uf lover voltage segment (25V not 450v) tested at 0.2uf. The rest tested OK but it's interesting that most of them were much higher values than expected. Nearly all of them were 43% to 60% higher than marked on the can.

The parts I replaced them with were as stated on the can.

This brings up a new issue.
Although the distortion is now gone and the amp sounds great, there is now (perhaps there always was) a 120hz hum with no input connected. From what I've read this implies a filter cap or too much ripple. Also, when the treble is increased too much (even with no input) it starts to oscillate at around 10K.

Here is an iPhone plot holding it in front of the speaker. The top is with the treble knob just before oscillation, and the bottom is turned up past that point.
hum and ossolation.jpg
Also, I found one of the mystery parts (the one in clear sleeve) was a dog bone resister. The other one (the two green parts in series) is still a mystery. I disconnected and leg and checked:
Diodes: Nope.
Resistors: Nope.
Capacitors: Who knows. They gave readings but quite different depending on polarity on my cap tester. Also, the reading for one part didn't match the other part.

A schematic sure would help. Any other ideas before I drag the scope out?
Tubes test fine but I've swapped in-and-out all tubes with spares except the 6V6 (I don't have any spares to try).

Bryan
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EpicenterBryan
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Re: Schematic / advice needed: Presto K11

Post: # 36888Unread post EpicenterBryan
Sun Aug 23, 2015 2:59 pm

This gets even more strange.
With no input connected. If I turn the amp upside down it will not oscillate even with treble all the way up.
If I put it back right side up, and turn it 90 degrees, it oscillates even worse at a lower treble setting.

I turned off the florescent lights, moved broadcasting devices to another room. Same thing. But there are other things like WiFi router - wireless cameras, wireless security system and all that stuff.

Bryan

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markrob
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Re: Schematic / advice needed: Presto K11

Post: # 36893Unread post markrob
Sun Aug 23, 2015 9:21 pm

Hi,

Nice job on the re-wiring.

Does the hum and/or oscillation go away if you short the input? Does the hum vary with the volume control? If so, its a grounding or shielding issue. If not try pulling all tubes except the rectifier and 6V6 output. If you still have hum, there may be an issue with the wiring of the filter caps. The rectifier could be bad as well. If your scope has an 10X probe and can be AC coupled, you can measure the AC ripple on the DC B+ bus. Should be under a couple of volts peak to peak at the first filter after the rectifier. You can also measure with a multimeter set to AC volts. If not, make sure you have the filter caps properly grounded.

As far as measuring with a meter goes, you won't find leakage problems with the paper caps since they are not polarized at hundreds of volts. Also, it is possible that some are ok or leakage does not matter in parts of the circuit. On the electrolytics, those cans typically had very large tolerances. +80% -20% was not uncommon. The value is not that critical and they tended to run high in value as you can see.

Mark

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EpicenterBryan
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Re: Schematic / advice needed: Presto K11

Post: # 36920Unread post EpicenterBryan
Tue Aug 25, 2015 12:14 am

markrob wrote: If not try pulling all tubes except the rectifier and 6V6 output. If you still have hum, there may be an issue with the wiring of the filter caps.
Once again, Mark nailed it.

Some how I swapped the first stage filter power (more dirty ... 2.6VAC ripple) to the 6V6 grid rather than cleaner 2nd stage filter power ( 0.2VAC ripple). Hum is nearly gone now that it's been corrected. If you get you face right in front of the speaker (where the head would connect) you hear it very slightly. I'll look at increasing the filter cap just because I can.

As far as the oscillation goes... It looks like there is a cold solder joint at an internal connection to the harness (not one of mine). Moving the amp orientation around actually moves the "AC in / head out" cable which moves the internal harness ever so slightly. It's easy to duplicate so I'll nail down which connection is the issue.

I've also discovered two other things. The Mic in connector is a goofy Switchcraft 2501 which is not grounded. Since the Mic input pot introduces noise if turned up and no mic is connected, I'll make a shorting plug to connect when not needed (or ever). But I'm thinking I should just replace the pot with a pot that also has an on-off switch built in so the input could just be shorted when not being used. Comments?
switchcraft microphone connector.jpg
Also, in messing with the beast's guts (don't do this at home - dangerous voltages in there) you can introduce noise just by waving your hands around the 12AX7 and the whole input stage in the open bottom chassis. I realize this gets mounted in the suitcase but do you think it's worth making a bottom cover (perhaps mesh screen for air flow)? Ultimately I may end up selling this when it's as good as it can be for the next guy. I just want it as clean and bullet proof as possible.

Imagine if that Mic input pot got bumped and how pissed off you would be with yourself (or me) when your cuts were full of noise!

Thoughts?

Bryan
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