Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bbmade on April 05, 2025, 04:40:04 pm
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UPDATE - I swapped the first dropping resistor and it did not help. Right now, the resistor values are back to the Mojotone specs from the schematic.
Here are some details:
Plate Voltage Outside 6V6 281.8 VDC
Plate Voltage Inside 6V6 285.1 VDC
V1A Plate 83.4 VDC
V1B Plate 82.6 VDC
V2A Plate 106.3 VDC
V2B Plate 132.5 VDC
Filter Node A 305.8 VDC
Filter Node B 222.1 VDC
Filter Node C 162.6 VDC
Output transformer resistance: Outer 331.8 ohms, Inner 295.42 ohms
Voltage drop measured the other day were: Outer 22.47, Inner 21.08
So, I am at a loss. The cathode resistor actual value is 264 ohms and there was about 40 volts across it tonight after putting everything back to the stock values.
I have a Mojotone 5E3 kit I built quite a while ago. I never really gelled with it and it never sounded “good” to me. I haven’t played it much over the years.
The last couple days I have tried to see if something is wrong, and came to the conclusion the bias was way too hot. With the provided 270 ohm cathode resistor for the 6V6’s, they were dissipating 16 watts.
I changed it to a 470 ohm 5 watt and it got down to 14 watts.
To get the tubes to dissipate at 9 or 10 watts it took a 1.2K 5 watt resistor but of course the cathode bypass cap couldn’t handle that. With the 1.2k resistor there was a little over 100 volts going through the cathode resistor.
I’ve checked all the resistor values in the amp to make sure they are correct and they are. I’ve tried another set of 6V6’s, same results.
I’m not sure what I am missing here. I had assumed because it’s a tried and true design that the bias would be fine, but something is amiss here.
I do have specific notes on plate voltage and voltage drops, including the output transformer resistance in the shop.
I’ve walked away from it for the day. I’ll continue doing my homework to try and figure out what I’ve got wrong.
Any insights are always appreciated.
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I do have specific notes on plate voltage and voltage drops
that will help, you probably have too high a B+ feeding the 6V6's
the specific schematic (NOT layout) will also help.
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What rectifier tube is in there?
What brand 6V6 tubes?
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I have a Mojotone 5E3 kit I built quite a while ago. ... The last couple days I ... came to the conclusion the bias was way too hot. With the provided 270 ohm cathode resistor for the 6V6’s, they were dissipating 16 watts.
I changed it to a 470 ohm 5 watt and it got down to 14 watts.
...
I’ve walked away from it for the day. I’ll continue doing my homework to try and figure out what I’ve got wrong. ...
You haven't done anything wrong. Most of the "5E3 Deluxe" power transformers out there deliver way too much voltage, and over-dissipate the 6V6s.
On another forum, someone tried several transformers from several vendors and ran into the same issue over & over.
Rather than do that, and easier path is to increase the resistor-value between the filter caps that feed the output transformer & 6V6 screens. It's normally 4.7kΩ or 5kΩ. Bump that up to about 15-18kΩ.
- This will reduce the screen voltage, which reduces the plate current.
- The lower screen voltage allows a smaller cathode resistor value for a given plate current.
- The lower cathode resistor value and smaller plate current will yield a smaller bias voltage.
- The smaller bias voltage makes the power section more-sensitive & easier to drive to max power & into distortion.
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5E3 schematic
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Mojotone 5E3 kit
kits have a tendency to not follow an original design, hence his specific schemo
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Mojotone 5E3 kit
kits have a tendency to not follow an original design, hence his specific schemo
You're absolutely right to ask for the schematic. In fact, there's a forum rule that says topic authors must publish the schematic.
My experience in announcing kits of specific models, mainly from Fender (here 5E3), has never shown me any notable differences.
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What rectifier tube is in there?
What brand 6V6 tubes?
5Y3 JJ, also tried TAD.
Tungsol, two different sets.
Edit; Untangled quote, Willabe
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Thank you, I uploaded the schematic on my original post.
Mojotone 5E3 kit
kits have a tendency to not follow an original design, hence his specific schemo
You're absolutely right to ask for the schematic. In fact, there's a forum rule that says topic authors must publish the schematic.
My experience in announcing kits of specific models, mainly from Fender (here 5E3), has never shown me any notable differences.
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Thank you, I uploaded the schematic on my original post.
Mojotone 5E3 kit
kits have a tendency to not follow an original design, hence his specific schemo
You're absolutely right to ask for the schematic. In fact, there's a forum rule that says topic authors must publish the schematic.
My experience in announcing kits of specific models, mainly from Fender (here 5E3), has never shown me any notable differences.
Almost same schematic than original Fender
The Mojotone uses a much quieter tubes heater circuit than the Fender.
On the other hand, the Mojotone schematic is very badly drawn. The Fender is much better.
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... With the provided 270 ohm cathode resistor for the 6V6’s, they were dissipating 16 watts. ...
Rather than / additional to, your result, please provide your input data, show your workings.
ie anode, cathode, screen grid, XA, XB and XC V DC at idle.
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the math based on schematic data indicates ~~~~ 12+W per tube
20.1 /270 = ~75mA /2 tubes = 37mA per tube
.037 * (363-20.1) = 12.7W - 2W for G2 12.5W
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the math based on schematic data indicates ~~~~ 12+W per tube
20.1 /270 = ~75mA /2 tubes = 37mA per tube
.037 * (363-20.1) = 12.7W - 2W for G2 12.5W
* = X
0.037 X ( 363 - 20,1) =12,7
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What rectifier tube is in there?
What brand 6V6 tubes?
5Y3 JJ, also tried TAD.
Tungsol, two different sets.
Edit; Untangled quote, Willabe
I find that modern reproduction 6V6GTs tend to be unreliable at higher voltages. Even the fancily labelled TADs are garbage.
Best thing you can do is get the voltage down to around 350 or change to JJ6V6S
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the math based on schematic data indicates ~~~~ 12+W per tube
20.1 /270 = ~75mA /2 tubes = 37mA per tube
.037 * (363-20.1) = 12.7W - 2W for G2 12.5W
* = X
0.037 X ( 363 - 20,1) =12,7
A lowercase 'x' or '*' is acceptable as a multiplication symbol, but not an uppercase 'X' https://en.m.wikipedia.org/wiki/Multiplication_sign
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* = X0.037 X ( 363 - 20,1) =12,7
I'm born, raised and educated in America, comma's and capital X are used in English class not math class :icon_biggrin:
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You're absolutely right to ask for the schematic. In fact, there's a forum rule that says topic authors must publish the schematic.
There's no rule that you have to post a schematic. :laugh:
We ask that you will, but it's not a rule.
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* = X0.037 X ( 363 - 20,1) =12,7
I'm born, raised and educated in America, comma's and capital X are used in English class not math class :icon_biggrin:
Get it right, it's edumacated. :laugh:
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:laugh:
that's for public school
Life is where education gets skool'd into ya.
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Updated my original post with some specs. I’ll be working on it again tomorrow so, if anyone happens to see this, as always, open to any suggestions.
Thank you
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UPDATE - I swapped the first dropping resistor and it did not help. Right now, the resistor values are back to the Mojotone specs from the schematic.
Here are some details:
Plate Voltage Outside 6V6 281.8 VDC
Plate Voltage Inside 6V6 285.1 VDC
V1A Plate 83.4 VDC
V1B Plate 82.6 VDC
V2A Plate 106.3 VDC
V2B Plate 132.5 VDC
Filter Node A 305.8 VDC
Filter Node B 222.1 VDC
Filter Node C 162.6 VDC
Output transformer resistance: Outer 331.8 ohms, Inner 295.42 ohms
Voltage drop measured the other day were: Outer 22.47, Inner 21.08
So, I am at a loss. The cathode resistor actual value is 264 ohms and there was about 40 volts across it tonight after putting everything back to the stock values.
Measure and post DC Volts at all pins of the 6V6s. Did you use any surplus/salvaged coupling caps?
I ask because you have "impossible current" for 6V6s with 300v plate and 200v screen, unless you have +35 volts at Pin 5 (with +40v at the cathode) or your 6V6s are defective.
The lower graph on Page 4 of the 6V6 data sheet (https://frank.pocnet.net/sheets/093/6/6V6GT.pdf) shows it takes just over -5v grid-to-cathode to get 60mA of plate current at 200v screen. The user graph on the same page shows the 6V6 can only manage 84mA total with 200v on the screen.
Somehow, your tubes are passing high plate current as though they're being driven. So there's either a serious wiring error, supremely leaky coupling caps, or defective 6V6s.
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UPDATE - I swapped the first dropping resistor and it did not help. Right now, the resistor values are back to the Mojotone specs from the schematic.
Here are some details:
Plate Voltage Outside 6V6 281.8 VDC
Plate Voltage Inside 6V6 285.1 VDC
V1A Plate 83.4 VDC
V1B Plate 82.6 VDC
V2A Plate 106.3 VDC
V2B Plate 132.5 VDC
Filter Node A 305.8 VDC
Filter Node B 222.1 VDC
Filter Node C 162.6 VDC
Output transformer resistance: Outer 331.8 ohms, Inner 295.42 ohms
Voltage drop measured the other day were: Outer 22.47, Inner 21.08
So, I am at a loss. The cathode resistor actual value is 264 ohms and there was about 40 volts across it tonight after putting everything back to the stock values.
Measure and post DC Volts at all pins of the 6V6s. Did you use any surplus/salvaged coupling caps?
I ask because you have "impossible current" for 6V6s with 300v plate and 200v screen, unless you have +35 volts at Pin 5 (with +40v at the cathode) or your 6V6s are defective.
The lower graph on Page 4 of the 6V6 data sheet (https://frank.pocnet.net/sheets/093/6/6V6GT.pdf) shows it takes just over -5v grid-to-cathode to get 60mA of plate current at 200v screen. The user graph on the same page shows the 6V6 can only manage 84mA total with 200v on the screen.
Somehow, your tubes are passing high plate current as though they're being driven. So there's either a serious wiring error, supremely leaky coupling caps, or defective 6V6s.
Thank you. I picked up a pair of 6V6’s today. I suspect this amp has cooked two sets so far and the use of the amp has been so infrequent, and so little time I never noticed how hot they were.
I will be working on this tomorrow and post that info.
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UPDATE - I swapped the first dropping resistor and it did not help. Right now, the resistor values are back to the Mojotone specs from the schematic.
Here are some details:
Plate Voltage Outside 6V6 281.8 VDC
Plate Voltage Inside 6V6 285.1 VDC
V1A Plate 83.4 VDC
V1B Plate 82.6 VDC
V2A Plate 106.3 VDC
V2B Plate 132.5 VDC
Filter Node A 305.8 VDC
Filter Node B 222.1 VDC
Filter Node C 162.6 VDC
Output transformer resistance: Outer 331.8 ohms, Inner 295.42 ohms
Voltage drop measured the other day were: Outer 22.47, Inner 21.08
So, I am at a loss. The cathode resistor actual value is 264 ohms and there was about 40 volts across it tonight after putting everything back to the stock values.
Measure and post DC Volts at all pins of the 6V6s. Did you use any surplus/salvaged coupling caps?
I ask because you have "impossible current" for 6V6s with 300v plate and 200v screen, unless you have +35 volts at Pin 5 (with +40v at the cathode) or your 6V6s are defective.
The lower graph on Page 4 of the 6V6 data sheet (https://frank.pocnet.net/sheets/093/6/6V6GT.pdf) shows it takes just over -5v grid-to-cathode to get 60mA of plate current at 200v screen. The user graph on the same page shows the 6V6 can only manage 84mA total with 200v on the screen.
Somehow, your tubes are passing high plate current as though they're being driven. So there's either a serious wiring error, supremely leaky coupling caps, or defective 6V6s.
JJ 6V6S (New)
Outside Tube
P1 0V
P2 0V
P3 292.6V
P4 216V
P5 4.95V
P6 5.01V
P7 0V
P8 39.1V
Inside Tube
P1 0V
P2 0V
P3 289.6V
P4 213V
P5 5.2V
P6 5.1V
P7 0V
P8 38.8V
OT Resistance - Outside 319.5 ohm, Inside 285 ohm
CT Voltage 308.5
Outside 308.5 - 292.6 = 15.9 Vdrop
Inside 308.5 - 289.6 = 18.9 Vdrop
15.9 / 319.5 = .0497
18.9 / 285 = .0663
So, I now think, this amp has cooked two different inner 6V6 tubes. The new JJ's show too much plate current on that inner tube. I am pondering getting rid of the shared cathode resistor and install one per tube, each with its own cathode bypass cap.
BUT....
IF I MEASURE VOLTAGE DROP DIRECTLY - Pin 8 of rectifier tube to each plate of the 6V6 I'm getting different voltage drops and the picture looks different.
Outside Vdrop = 20.7
Inside Vdrop = 18.5
20.7 / 319.5 = .0657
18.5 / 281.13 = .0658
This looks like I would not need to have separate cathode resistors.
I swear, there's nothing quite like working on amps, to make me feel like a moron. I also wouldn't be surprised if there are mistakes in my math.
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P5 5.2V
There’s your issue!
Why oh why aren't the control grids at 0V DC?
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P5 5.2V
Output tube sockets are wired like the diagram. No 470 ohm resistors, just the 1.5K.
At the moment, I have a 5 watt, 470 ohm resistor for the cathodes of the 6V6’s with no cathode bypass cap. It’s biased at 13 watts dissipation and has around 66 volts across that resistor.
I don’t know what to do about Pin 5, any advice is welcome. Next chance to work on it will be Sunday or maybe Monday.
There’s your issue!
Why oh why aren't the control grids at 0V DC?
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P5 5.2V
Output tube sockets are wired like the diagram. No 470 ohm resistors, just the 1.5K.
At the moment, I have a 5 watt, 470 ohm resistor for the cathodes of the 6V6’s with no cathode bypass cap. It’s biased at 13 watts dissipation and has around 66 volts across that resistor.
I don’t know what to do about Pin 5, any advice is welcome. Next chance to work on it will be Sunday or maybe Monday.
There’s your issue!
Why oh why aren't the control grids at 0V DC?
Coupling caps??
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What do you measure at these points?
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Neglecting to ground the 6V6 grid leaks is a common error.
What the resistance between the 220k junction and ground (amp unplugged from mains and deenergised).
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^What pdf64 said ^
Use your R meter to check there is zero Ohms DC-continuity between the 220k grid leak resistors and chassis ground for the 6V6s. You might have forgotten to install the wire from there to the ground.
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Well, once again, the forum, and the wealth of knowledge shared here, has helped immensely. I absolutely forgot that jumper from the 220K resistors. When measuring all the resistors for tolerance I noticed those had strange values and mistakenly assumed it was how they were wired in the circuit. I should have taken the clue and dug a little deeper.
The voltages certainly tell a story, if I were smart enough to slow down and document all of them. I still don't think I would have caught that mistake in any reasonable period of time so, thank you to everyone who responded and helped.
Now...
Outside 6V6
P1 0V
P2 0V
P3 361V
P4 330V
P5 .025V
P6 .025V
P7 0V
P8 22.26V
Inside 6V6
P1 0V
P2 0V
P3 361.5V
P4 329V
P5 .026V
P6 .028V
P7 0V
P8 22.25V
CT Voltage at Rectifier 377.2V
Plate Outside 6V6 364.8V
Plate Inside 6V6 366V
Voltage Drop Pin 8 to Pin 3 of each
Outside 12.4V
Inside 11.28V
12.4 / 312.2 = 39.7mA
11.28 / 287.65 = 40.4mA
THE AMP SOUNDS SO MUCH BETTER. So much yet to learn, thank you all again.