I purchased a Pilot 248B Integrated Amp 2 years ago. I fixed some bad components re-tubed it and set the bias according to the manual. The amp worked fine and produced about 30 watts per channel into 8 ohms at clipping.
I gave it to a friend. He contacted me recently because it began distorting badly at power levels above one watt.
I can't find a schematic for the 248B. The attached schematic is for a 264 amp but is similar enough from V3 and V4 thru to the outputs for troubleshooting.
I replaced all the caps in the HV supply and the 100uf cap in the bias supply. The HV supply shows a 10k ohm 2W resistor before V3 and V4. It was bad. In my amp it is actually a 6700 ohm resistor. I replaced it with a 10k 5w resistor.
These changes brought one channel back to 30w output before clipping and the other channel back to ~12w before clipping. I can hear a low level buzz in the bad channel that does not exist in the good channel. Otherwise the sound is good until clipping.
I swapped the 7591s and the 12ax7 amp/pi tubes between the two channels and the problem stayed with the bad channel. I did other tests on the tubes and I think they are all okay.
The 248B has a 100k ohm resistor in series with the input and the grid of each of V3 and V4 audio amplifier stages. The signal before and after the resistor in the V3 channel does not clip up to full power. In the v4 channel it does not clip before the resistor but it shows clipping after the resistor at about 10v peak.
In the V3 channel I replaced the 4700 ohm cathode resistor (it was 5600 ohms on my meter), the 100k ohm plate resistor (120k on the meter), and the coupling cap (it is actually 0.1uf). This did not fix the problem.
Can someone here suggest what I should do next to identify and fix the problem.
Thanks,
Steve
I gave it to a friend. He contacted me recently because it began distorting badly at power levels above one watt.
I can't find a schematic for the 248B. The attached schematic is for a 264 amp but is similar enough from V3 and V4 thru to the outputs for troubleshooting.
I replaced all the caps in the HV supply and the 100uf cap in the bias supply. The HV supply shows a 10k ohm 2W resistor before V3 and V4. It was bad. In my amp it is actually a 6700 ohm resistor. I replaced it with a 10k 5w resistor.
These changes brought one channel back to 30w output before clipping and the other channel back to ~12w before clipping. I can hear a low level buzz in the bad channel that does not exist in the good channel. Otherwise the sound is good until clipping.
I swapped the 7591s and the 12ax7 amp/pi tubes between the two channels and the problem stayed with the bad channel. I did other tests on the tubes and I think they are all okay.
The 248B has a 100k ohm resistor in series with the input and the grid of each of V3 and V4 audio amplifier stages. The signal before and after the resistor in the V3 channel does not clip up to full power. In the v4 channel it does not clip before the resistor but it shows clipping after the resistor at about 10v peak.
In the V3 channel I replaced the 4700 ohm cathode resistor (it was 5600 ohms on my meter), the 100k ohm plate resistor (120k on the meter), and the coupling cap (it is actually 0.1uf). This did not fix the problem.
Can someone here suggest what I should do next to identify and fix the problem.
Thanks,
Steve
There must be cold soldering joints...
Also check the input connector contact, especially the ground contact.
Does anyone have a complete Pilot 248 schematic?
I need trouble-shooting this amp as well.
By the way where did you get the aluminum can capacitor replacement?
I appreciate your support.
Also check the input connector contact, especially the ground contact.
Does anyone have a complete Pilot 248 schematic?
I need trouble-shooting this amp as well.
By the way where did you get the aluminum can capacitor replacement?
I appreciate your support.
Measure the voltages on pins 1 and 6 of the double triodes. That should give you a clue also I would check the output valve voltages by comparison.
Hi there,
Did you fix the amp yet?
I hooked my 248 amp up with Rogers LS3/5a yesterday.
It sounded wonderful....
It is well worth to fix it.
Did you fix the amp yet?
I hooked my 248 amp up with Rogers LS3/5a yesterday.
It sounded wonderful....
It is well worth to fix it.
Update
I had a lot of free time last year so I pulled this amplifier off the shelf and attempted repairs again.
The output transformer in the weak channel was bad.
The pilot 248B uses a fixed bias output stage. I had not rebiased them since I gave him the amplifier. He said he played it several hours a day for a couple of years.
I think as the tubes aged one started to hog current, red plated, and took out the output transformer.
Transcendar offered to put one of his cores in the transformer if I was willing to modify the circuit. I hope to get that done this spring.
Steve
I had a lot of free time last year so I pulled this amplifier off the shelf and attempted repairs again.
The output transformer in the weak channel was bad.
The pilot 248B uses a fixed bias output stage. I had not rebiased them since I gave him the amplifier. He said he played it several hours a day for a couple of years.
I think as the tubes aged one started to hog current, red plated, and took out the output transformer.
Transcendar offered to put one of his cores in the transformer if I was willing to modify the circuit. I hope to get that done this spring.
Steve
7591 with fixed bias g1 maximum DC return resistance is 300k.
Rg is 270k in that amp. Be sure to use a Robust 7591.
With the extra DC return resistance of the fixed adjustable bias supply, you have to add that resitance to the 270k Rg.
Lesser 7591 will be Bust-ed, not Robust.
I never liked fixed bias, especially when both the push and pull output tubes have exactly the same bias voltage, like this amp has. Output transformers hate un-matched plate currents.
Non Individual fixed bias pots is terrible unless the tubes are very well matched for that amplifier.
In the old days, sometimes there were very well matched tubes available from the manufacturer (matched for the particular amplifier model).
If I had that amplifier, I would "ruin it" for some people.
Individual Self Bias resistors, and individual bypass caps (4 Rs, 4 Cs, for Stereo).
Then return the 270k Rg resistors to ground.
"Ruin it" = a little Less Power, not as good performance on the Telarc recording of the 1812 Overture, with the 6Hz canon . . .
. . . But, More Reliable, and better match of plate current for most good output tubes.
By the way, putting a pot in the concertina only solves one problem . . . it makes up for un-equal AC gain of the output tubes.
And a pot wiper does not add reliability, it takes it away.
With so much global negative feedback, it covers most of the problems like un-equal AC gain, anyway.
Tradeoffs.
Just My Opinions.
Your Milage May Vary.
Rg is 270k in that amp. Be sure to use a Robust 7591.
With the extra DC return resistance of the fixed adjustable bias supply, you have to add that resitance to the 270k Rg.
Lesser 7591 will be Bust-ed, not Robust.
I never liked fixed bias, especially when both the push and pull output tubes have exactly the same bias voltage, like this amp has. Output transformers hate un-matched plate currents.
Non Individual fixed bias pots is terrible unless the tubes are very well matched for that amplifier.
In the old days, sometimes there were very well matched tubes available from the manufacturer (matched for the particular amplifier model).
If I had that amplifier, I would "ruin it" for some people.
Individual Self Bias resistors, and individual bypass caps (4 Rs, 4 Cs, for Stereo).
Then return the 270k Rg resistors to ground.
"Ruin it" = a little Less Power, not as good performance on the Telarc recording of the 1812 Overture, with the 6Hz canon . . .
. . . But, More Reliable, and better match of plate current for most good output tubes.
By the way, putting a pot in the concertina only solves one problem . . . it makes up for un-equal AC gain of the output tubes.
And a pot wiper does not add reliability, it takes it away.
With so much global negative feedback, it covers most of the problems like un-equal AC gain, anyway.
Tradeoffs.
Just My Opinions.
Your Milage May Vary.
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6A3sUMMER thanks for the reply.
The bias scheme on this amp is a PITA to adjust.
The maximum bias voltage is too low. It barely makes 20 volts on this amp. With some tubes that I tried it was insufficient to bias or achieve balance. In addition a bias change on any output tube affects the others in this amp.
I bought this amp as a present for my friend. I replaced it with a SS amp and he is happy.
I do not intend to keep it. My plan is to fix it and sell as is.
I like your concept about cathode bias and may change to that before I sell it. That will depend in part on problems I may have when biasing new tubes.
Steve
Schematic of the 248B is attached.
The bias scheme on this amp is a PITA to adjust.
The maximum bias voltage is too low. It barely makes 20 volts on this amp. With some tubes that I tried it was insufficient to bias or achieve balance. In addition a bias change on any output tube affects the others in this amp.
I bought this amp as a present for my friend. I replaced it with a SS amp and he is happy.
I do not intend to keep it. My plan is to fix it and sell as is.
I like your concept about cathode bias and may change to that before I sell it. That will depend in part on problems I may have when biasing new tubes.
Steve
Schematic of the 248B is attached.
Part of the problem is that the B+ is not regulated (not a problem for individual self bias).
What happens is that you adjust the bias of a push pull pair at one time for the proper current. That raises or lowers the B+ voltage.
Then, you adjust the other channel's push pull pair current, and that raises or lowers the B+.
Now, you have to go back and adjust the first push pull pair bias to get the proper current; and then re-check the second pair push pull current.
I remember when I had to do this on a Dyna Stereo 70.
Two pots for 4 tubes.
Uggh! I am sorry, I do not know of an international word equivalent for Uggh!
What happens is that you adjust the bias of a push pull pair at one time for the proper current. That raises or lowers the B+ voltage.
Then, you adjust the other channel's push pull pair current, and that raises or lowers the B+.
Now, you have to go back and adjust the first push pull pair bias to get the proper current; and then re-check the second pair push pull current.
I remember when I had to do this on a Dyna Stereo 70.
Two pots for 4 tubes.
Uggh! I am sorry, I do not know of an international word equivalent for Uggh!
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Thanks for that explanation about adjusting fixed bias on multiple tubes.
I have a GTA SE-40. It is a PSE with 3 6L6GC types per channel in fixed bias. Adjust one tube and you may have to readjust all 5 other tubes.
I mashed up the device in the attachment so that I could see the changes on all tubes at the same time. Biasing is still a see-saw approach but much easier than when I couldn't see results on all tubes at the same time. Total cost was only a few dollars.
Steve
I have a GTA SE-40. It is a PSE with 3 6L6GC types per channel in fixed bias. Adjust one tube and you may have to readjust all 5 other tubes.
I mashed up the device in the attachment so that I could see the changes on all tubes at the same time. Biasing is still a see-saw approach but much easier than when I couldn't see results on all tubes at the same time. Total cost was only a few dollars.
Steve
Attachments
ArcticBrew,
A. The Pilot amp you want to fix and sell . . .
1. If you go to individual self bias, you will loose power,
but the person who purchases it will never have to go through the trouble of adjusting the bias ever again. It will run for years. He will be happy.
2. If you retain the poorly designed fixed adjustable bias, the first time the new owner has a failure, or has to adjust the bias, he will be mad at you.
And he will sell that amplifier and pass the problem on to someone else.
B. I knew someone who had one of those Golden Tube Amp SE-40.
I have never seen the schematic, but even without seeing it, I bet I would go to 6 individual self bias resistors, and 6 bypass caps. I would fit them in there somehow.
Then I would enjoy listening to the now slightly lower power amp, and I would have one that was more reliable, and never needing adjustment.
And, any repair shop that did not know how to repair an amplifier thats only modification was changing from fixed adjustable bias to individual self bias, probably ought not be allowed to take the bottom cover off.
Just my opinions.
A. The Pilot amp you want to fix and sell . . .
1. If you go to individual self bias, you will loose power,
but the person who purchases it will never have to go through the trouble of adjusting the bias ever again. It will run for years. He will be happy.
2. If you retain the poorly designed fixed adjustable bias, the first time the new owner has a failure, or has to adjust the bias, he will be mad at you.
And he will sell that amplifier and pass the problem on to someone else.
B. I knew someone who had one of those Golden Tube Amp SE-40.
I have never seen the schematic, but even without seeing it, I bet I would go to 6 individual self bias resistors, and 6 bypass caps. I would fit them in there somehow.
Then I would enjoy listening to the now slightly lower power amp, and I would have one that was more reliable, and never needing adjustment.
And, any repair shop that did not know how to repair an amplifier thats only modification was changing from fixed adjustable bias to individual self bias, probably ought not be allowed to take the bottom cover off.
Just my opinions.
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Maybe worth considering: a 1dB change in overall volume can be noticed if changed rapidly, but just. But a 1dB change in peak output capacity, in a well designed amplifier, is extremely hard to hear, and that's the order of change that cathode bias makes vs. "fixed" bias in dynamic conditions.
The importance of Watts is a sad leftover from the HiFi Wars of the past. DIYers don't have to accept those old FUDD goals.
YOS,
Chris
The importance of Watts is a sad leftover from the HiFi Wars of the past. DIYers don't have to accept those old FUDD goals.
YOS,
Chris