I think the screen ought to be higher than 22v myself. R4 is supposed to be 1.2M, is it?
A lot of times small residual voltages can mess up meter readings, particularly on high value resistors. measure the resistance one way and then reverse the probe leads and remeasure. if you are measuring resistance, the readings will be the same. If the resistance readings are substantially different, then there is voltage in the circuit. it only takes a tiny bit of voltage to do this.
Another check on your methods is this: remember your meter has an impedance itself, it is not transparent. So its internal resistance and R4 will form a voltage divider. Measure voltage to ground from pin 6 of the 12AU6, you got 22v. recheck just for fun. Now measure the voltage across R4. Not to ground, but end to end. You know voltage B was 120v, but recheck. Now we know the voltage across R4. Subtract that from voltage B, does it leave 22v?
Now look at that tube another way. Voltage B is 120v. The plate voltage at pin 5 is also 120v. I assume there is really a small difference, maybe a fraction of a milliamp. That means almost zero current is flowing through R5, meaning almost no current is flowing through that tube. Just more evidence, and supporting possible low screen voltage as a diagnosis.
One way to turn off a pentode is to remove voltage from its screen. 22v in place of closer to 100 is like turning down the tube.
If that tube is not conducting, then it cannot amplify.
A lot of times small residual voltages can mess up meter readings, particularly on high value resistors. measure the resistance one way and then reverse the probe leads and remeasure. if you are measuring resistance, the readings will be the same. If the resistance readings are substantially different, then there is voltage in the circuit. it only takes a tiny bit of voltage to do this.
Another check on your methods is this: remember your meter has an impedance itself, it is not transparent. So its internal resistance and R4 will form a voltage divider. Measure voltage to ground from pin 6 of the 12AU6, you got 22v. recheck just for fun. Now measure the voltage across R4. Not to ground, but end to end. You know voltage B was 120v, but recheck. Now we know the voltage across R4. Subtract that from voltage B, does it leave 22v?
Now look at that tube another way. Voltage B is 120v. The plate voltage at pin 5 is also 120v. I assume there is really a small difference, maybe a fraction of a milliamp. That means almost zero current is flowing through R5, meaning almost no current is flowing through that tube. Just more evidence, and supporting possible low screen voltage as a diagnosis.
One way to turn off a pentode is to remove voltage from its screen. 22v in place of closer to 100 is like turning down the tube.
If that tube is not conducting, then it cannot amplify.
This is a new resistor and - R4 is reading 1.299M Ohm
The original resistor that was there read 1.3M
Resistors have tolerances. If the resistor has no 4th band, it is 20%. If the 4th band is silver, it's 10% and gold is 5%. This tolerance can be either + or - the value of the resistor. So if you have a 1.2 meg, you can have 120k ohms in either direction of 1.2 meg for it to still be in tolerance. So your 1.2 meg can be as high as 1.32 meg or as low as 1.08 meg if the 4th band is silver. So that resistor was likely fine at 1.3 meg. Also, the difference between 1.3 meg and 1.299 meg is only 1000 ohms. For this application, this is virtually the same resistance.
Something is keeping that tube from conducting and I agree totally with Enzo. Get the screen voltage up. I would try to at least try to get it to at least 45-50 volts if you can. Maybe try lowering the resistance to about half of the 1.2 meg. If you have the correct resistors on hand, you can temporarily parallel another on to avoid all the soldering. If you parallel another 1.2 meg, you will cut it in half.
Something is keeping that tube from conducting and I agree totally with Enzo. Get the screen voltage up. I would try to at least try to get it to at least 45-50 volts if you can. Maybe try lowering the resistance to about half of the 1.2 meg. If you have the correct resistors on hand, you can temporarily parallel another on to avoid all the soldering. If you parallel another 1.2 meg, you will cut it in half.
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Thanks. I will try it. I have a lot of them. I also have new capacitors to cover all the caps in this amp and the H400A X 4. Once I get it working I plan on replacing them one at at time just in case I screw up I can go back to the last one I installed. 🙂 Plus I love hording parts.
I have been using a discharge stick with 2k Ohm resistor to drain capacitors each time I go in to work on it. I do not see and sparks. I wonder if it is necessary on this small amp? But I just feel better if I do.
I have been using a discharge stick with 2k Ohm resistor to drain capacitors each time I go in to work on it. I do not see and sparks. I wonder if it is necessary on this small amp? But I just feel better if I do.
I think we have narrowed it down to some point(s) between V1 and V2. I feel we on the verge of a major breakthrough. It's funny how I can get so much joy out of something so simple.
I thought you had already replaced caps, sorry.
The screen of the input pentode has a bypass cap to ground, if it were leaky it would drag down the screen voltage. In fact, just disconnect one end of it and see if the voltage comes up.
The screen of the input pentode has a bypass cap to ground, if it were leaky it would drag down the screen voltage. In fact, just disconnect one end of it and see if the voltage comes up.
I see 96 V DC across R4. Minus B leaves 24 volts.
I removed one end of C2 now it read 22.22 V DC.
I paralleled five 2.1m 's in there on pin 6 to get 38V DC and 116V DC at 'B'. buzz is a lot louder when I finger test but no guitar sounds.
So for grins I (in series) strung a few 390K resistors there and got 99v dc at pin 6 and 104 at 'B'. the finger buzz got really loud but still no guitar signal. even Pin 1 on V1 gave a buzz.
So for grins I (in series) strung a few 390K resistors there and got 99v dc at pin 6 and 104 at 'B'. the finger buzz got really loud but still no guitar signal. even Pin 1 on V1 gave a buzz.
I ran three resistors in series to ground from "B" so I could tap into the second point ( 67V dc) for pin 6 and noticed a large voltage drop.
Then I put it back to normal and disconnectrd R4 from pin 6, I saw 106v dc, when I reconnect the voltage drops to 22.
Is the voltage drop normal?
Then I put it back to normal and disconnectrd R4 from pin 6, I saw 106v dc, when I reconnect the voltage drops to 22.
Is the voltage drop normal?
I have a lot more free time now - so i am going back in to see if I can get this amp working one way or another. 🙂
I looked at schematics for amps with the same three tubes and it looks like 22 volts is ok on V1 (12AU6) pin 6.
After checking the resistance in the output transformer I found it to be 265 Ohms. This is causing the plate (pin 7) to be lower than the screen (pin 6) on V2 (50C5).
So.. Next I will replace the OT with a OT8SE from Musical Power Supplies.
After checking the resistance in the output transformer I found it to be 265 Ohms. This is causing the plate (pin 7) to be lower than the screen (pin 6) on V2 (50C5).
So.. Next I will replace the OT with a OT8SE from Musical Power Supplies.
> causing the plate (pin 7) to be lower than the screen (pin 6) on V2 (50C5).
This is quite normal. This is not a True Tetrode but some form of Pentode. Not a problem.
> 22v on the {6AU6} screen seems low. Generally from what I've seen, it's higher than that
If the 6AU6 must deliver "power", yes, G2 must be higher.
Here the power required is very-very low, however we want a lot of gain. Lower G2 (with other adjustments) is higher gain. The low-limit is that Vg2/Mu(g2) should be as-large as the largest G1 signal voltage. 22V/36 is 0.6V, guitar rarely gets quite that high, G2 voltage is fine.
> discharge stick with 2k Ohm resistor ... I do not see and sparks.
Amps like this, the residual glow of the cathode, and large currents against not-large filter caps, it probably self-discharges 90% in a second or so. Nevertheless DO use the discharge stick EVERY time.
> narrowed it down to some point(s) between V1 and V2
Isn't much there. Replaced C3? Strapped the top of Vol pot to 50C5 grid (bypassing volume control)?
This is quite normal. This is not a True Tetrode but some form of Pentode. Not a problem.
> 22v on the {6AU6} screen seems low. Generally from what I've seen, it's higher than that
If the 6AU6 must deliver "power", yes, G2 must be higher.
Here the power required is very-very low, however we want a lot of gain. Lower G2 (with other adjustments) is higher gain. The low-limit is that Vg2/Mu(g2) should be as-large as the largest G1 signal voltage. 22V/36 is 0.6V, guitar rarely gets quite that high, G2 voltage is fine.
> discharge stick with 2k Ohm resistor ... I do not see and sparks.
Amps like this, the residual glow of the cathode, and large currents against not-large filter caps, it probably self-discharges 90% in a second or so. Nevertheless DO use the discharge stick EVERY time.
> narrowed it down to some point(s) between V1 and V2
Isn't much there. Replaced C3? Strapped the top of Vol pot to 50C5 grid (bypassing volume control)?
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Thanks PRR,
I did replace C3.
I am getting signal (280 +/- mv AC guitar signal) going into V2 (50C5) but the signal does not make it to Pin 7. I get a steady 2 v at Pin 7. This made me suspect the OT.
So I pulled Tubes and checked the OT resistance (265 Ohms). Is this within spec?
Yep - I always use a discharge stick. Safety first, middle and last. I spent 19 years in HSE. That was fun. Safety is actually second. Danger was there first.
My moto was "don't tell - teach."
I take safety very seriously. Safety is not a laughing matter like a lot of our employees think. It is only funny until someone gets hurt - then its just hilarious. Not.
I did replace C3.
I am getting signal (280 +/- mv AC guitar signal) going into V2 (50C5) but the signal does not make it to Pin 7. I get a steady 2 v at Pin 7. This made me suspect the OT.
So I pulled Tubes and checked the OT resistance (265 Ohms). Is this within spec?
Yep - I always use a discharge stick. Safety first, middle and last. I spent 19 years in HSE. That was fun. Safety is actually second. Danger was there first.
My moto was "don't tell - teach."
I take safety very seriously. Safety is not a laughing matter like a lot of our employees think. It is only funny until someone gets hurt - then its just hilarious. Not.
> OT resistance (265 Ohms). Is this within spec?
Probably. "Wrong" would be infinity or few-Ohm. "Typical" would be 10% of nominal audio impedance. "Cheap typical" may be 15% of DCR. So 265 is not-wrong for a probable 1.5K-3K winding.
> signal (280 +/- mv AC guitar signal) going into V2 (50C5)
We need like 8V peak here to get the full output. But 0.28V should be audible.
> steady 2 v at Pin 7.
Audio or DC??
2V of DC is just wrong, find where your 130V gets lost.
2V of "audio"-- well I would expect 2V of AC ripple here. Can you tell 60Hz buzz from 440Hz guitar/test tone?
Clear all shorts from the output jack and speaker. I have hammered many "internal problems" which turned out to be shorted jack or cord.
OT secondary DC resistance is liable to be near 1 Ohm (often 10% of nominal 4 Ohm load) so it is hard to tell.
I don't know why I dislike thinking the OT is dead. I have seen too many techs rush to replace it, and high shipping costs, when that turned out to NOT be the problem. In this case it is not-quite the cheapest OT ever made, and I could give you two slightly cheaper 50C5 OTs if you came to my house. Problem is that dead radios (this is an AC/DC radio, tunerless, with full PT isolation) are no longer so common (even at my house). Hammond will sell you a much nicer iron but price with shipping like 7 times what the whole Kay/Harmony was worth (in 1962 dollars).
Probably. "Wrong" would be infinity or few-Ohm. "Typical" would be 10% of nominal audio impedance. "Cheap typical" may be 15% of DCR. So 265 is not-wrong for a probable 1.5K-3K winding.
> signal (280 +/- mv AC guitar signal) going into V2 (50C5)
We need like 8V peak here to get the full output. But 0.28V should be audible.
> steady 2 v at Pin 7.
Audio or DC??
2V of DC is just wrong, find where your 130V gets lost.
2V of "audio"-- well I would expect 2V of AC ripple here. Can you tell 60Hz buzz from 440Hz guitar/test tone?
Clear all shorts from the output jack and speaker. I have hammered many "internal problems" which turned out to be shorted jack or cord.
OT secondary DC resistance is liable to be near 1 Ohm (often 10% of nominal 4 Ohm load) so it is hard to tell.
I don't know why I dislike thinking the OT is dead. I have seen too many techs rush to replace it, and high shipping costs, when that turned out to NOT be the problem. In this case it is not-quite the cheapest OT ever made, and I could give you two slightly cheaper 50C5 OTs if you came to my house. Problem is that dead radios (this is an AC/DC radio, tunerless, with full PT isolation) are no longer so common (even at my house). Hammond will sell you a much nicer iron but price with shipping like 7 times what the whole Kay/Harmony was worth (in 1962 dollars).
A 120V:6V 0.5+A filament transformer will make a workable OT for this amp. Far from ideal, but if that works lame and what you got works not at all, it does lead to a new OT.
To test the output transformer you can feed the primary with an AC source, let say 24 volts AC, then measure the voltage on the secondary.
Then you can find the turns ratio and the impedance reflected in the primary for a given secondary load, for instance a 8 Ohm speaker
The formula is pretty simple just google about output transformer calculating turns ratio and impedance.
Sorry but what is the actual problem with your amp, is so simple very few things can go wrong.
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