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Old January 14th, 2013, 09:21 AM   #1 (permalink)
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Music Master bass amp filter cap confusion...

Ok I got two MMB 6v6gt amps both 1977 models one has the original filter caps at .70@350volts, the other has replacements at .22@350 volts.
The .70 one is much louder and breaks up quicker, the other is not as loud and breaks up much later. The schematic shows they should be .20@350volts. I did see a pic in this club of a mallory filter cap and it did say .20 on it. What have you guys seen in these? I like the tone of the .22 one best.

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Old January 14th, 2013, 10:32 AM   #2 (permalink)
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The ones I've seen use a 20/20/20-300v multisection. It's a 300v version of the cheapo multisection seen in '60s "transformerless" radios and phonographs.

"Not as loud and breaks up later" sounds like a weak 12AX7. All else bein' equal level the playing field by swapping the 12AX7 into the other amp.
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Old January 14th, 2013, 08:18 PM   #3 (permalink)
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Swapped all tubes back and forth no difference. My local amp Guru suggests to go with the .22 @350 then compare. Will do that next.
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Old January 15th, 2013, 05:04 PM   #4 (permalink)
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Well lets start with if the caps in question really are .7 and .22Uf, then they are not power supply filter caps.
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Old January 15th, 2013, 11:37 PM   #5 (permalink)
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oops! I am dangerous with decimal places...Make that 22mfd@350volts and 70mfd@350 volts. The one with the 22's is an earlier amp A851714, the later amp A965028 has the 70's in it. Both are the 6V6GT models. I Will pick up new 22's (maybe bump to 33's?) for both amps and then see if they sound more alike.
Thanks
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Old January 15th, 2013, 11:41 PM   #6 (permalink)
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Yeah, it could be an issue of cap leakage dropping the voltage to one.
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Old January 20th, 2013, 09:46 PM   #7 (permalink)
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Ok here is the latest...I changed the filter caps on both amps to 33mfd@350. One amp still louder than the other (same tubes, same speaker connected for all tests). Tested voltages on the input transformers on both amps and they match at 245 volts. I started taking readings off the 6v6gt tubes. Both amps have these voltages: pin8/cathode=18v(ok), pin3/plate=302v(ok), pin4/screengrid=297(ok), pin5/controlgrid=395V(ok on quieter amp, I cant get a steady reading on the louder amp) What could be up with pin 5? That is connected to the phase inverter transformer. voltages into the transformers agree with the schematic...Kinda reaching the end of my troubleshooting skills here...
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Old January 21st, 2013, 06:46 PM   #8 (permalink)
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I will assume that that is really 39.5v and not 395v. the problem with the other one is getting a good connection, If there were an unsteady voltage on the control grid, it would be amplified and you would hear it from the speaker
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Old January 21st, 2013, 11:05 PM   #9 (permalink)
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Just rechecked my voltages again...maybe I messed up on pin 5 the first time, but both amps are now giving me a fluctuating reading from 11-15 volts on pin 5. No idea what that reading should really be. I see it goes to higher volts at higher volume.
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Old January 23rd, 2013, 10:31 PM   #10 (permalink)
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Decided that there are a few caps left on each board I could change out. That must surely fix my situation. Will take care of it this weekend and report back.
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Old January 24th, 2013, 09:26 AM   #11 (permalink)
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I may have found the problem!
On the schematic it shows the vol pot as 250K linear, on the tone pot it shows as a 1meg audio pot.

My correct sounding amp has the vol=1meg /tone=250k (oldest amp)
My wrong sounding amp has the vol=250K/tone=1meg (newer amp)

Can anyone confirm what they have for pots in their 6v6gt MMB amp please?
Thanks
Rob
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Old January 26th, 2013, 04:08 PM   #12 (permalink)
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Musicmaster bass amp now works correctly!

I will post this here and in the MMB amp club pages so if anyone encounters this problem, they will know how to fix it. The problem was one MMB amp I had would go to full volume at around 3 on the volume dial, then distort. I recently got a second one and it worked properly and the volume control worked thru its full range. Both are from 1977 and both are 6v6gt models.

It must have been a bad day at the factory for my problem MMB amp.
They had put the volume pot in the tone hole and the tone pot in the volume hole! The volume pot should be a 1 Meg audio pot, the tone control should be a 250K linear pot! What threw me off was the schematic I found is wrong, it shows the pots reversed. Someone drew a layout from that and included that same mistake! Additionally Fender put in 70mfd filter caps instead of 20mfd filter caps in the bad amp! I put in the correct caps, swapped the volume and tone controls and rewired them. I also noticed on the good amp it had a 1K5 watt resistor in it but the bad one had a 1K 1/2 watt resistor. I went ahead and put in a 5W one, never hurts to go bigger there. These did not come with fuses so I added an inline fuse in the chassis with a 1 amp slow blow fuse in them. I put a sticker on the back explaining this. I have included some pics of the finished amps. They are louder and fuller sounding than Champ amps and start doing the tube breakup around 7-8 on the volume control.
You will notice the fixed one has a much more efficient speaker in it and it sounds really good. You could play bass thru the amp now as well as guitar and they would both sound good. Rob
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Old January 26th, 2013, 04:21 PM   #13 (permalink)
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Would be an easy mistake to make.

I love it when a thread has a happy ending and the op takes the time to report it.

Thanks Robster!
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Old January 26th, 2013, 04:24 PM   #14 (permalink)
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Quote:
Originally Posted by onenotetom View Post
Would be an easy mistake to make.

I love it when a thread has a happy ending and the op takes the time to report it.

Thanks Robster!
+1

Good work all around.
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