Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mebond11 on November 07, 2020, 11:56:57 am
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Hi folks,
My next project is the 5879 Blues Amp … I am attaching the schematic and a pdf of the layout I created in DIY if anyone's wants to look at it. I had a few questions about the circuit:
1. The wiring of the middle pot of the tone stack is confusing me...the ground terminal is jumpered to the wiper, which would mean the pot is zero resistance full up...shouldn't that be reversed? Would I be better off wiring it as a traditional fender tone stack?
2. I am thinking of putting in a 6BM8 pentode/triode in lieu of the 12AX7 for the reverb circuit, in order to increase gain and the amount of reverb. I see a number of different circuit examples for this...does someone have a proven circuit I can drop into this build? Does the substitution materially increase the reverb?
3. The schematic shows 1 ohm resistors between the cathode bypass cap and the cathodes of the power tubes. I haven't seen this before. What is the purpose of these resistors?
4. Anyone have any improvements they would suggest to this amp before I get going?
Thanks in advance for your help! I appreciate it very much.
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> 1 ohm ... What is the purpose of these resistors?
What is the usual purpose of 1 Ohm resistors in a tube amp?
Yes, they are connected a little different than usual but are used the same.
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3. The schematic shows 1 ohm resistors between the cathode bypass cap and the cathodes of the power tubes. I haven't seen this before. What is the purpose of these resistors?
It is for easy read tube's curent for tubes bias purpose.
Millivolts across a 1 ohms = milliamps
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Thanks...another question...the PT specified in the schematic is the 270FX which has a 550V secondary..although the circuit specifies 300V on each side of the center tap. Do I need the 272FX instead, which has a 600V secondary?
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I would recommend a PT with 300-0-300 instead of 275-0-275.
While it would work either way, I think I'd have the passive effects loop go into the reverb instead of the reverb into the passive effects loop.
The schematic should have the reverb tube labeled V3 instead of V2. The second gain stage with cathode follower is V2
On V2-3 with the 5uf cathode cap. IF the 5uf makes it too grainy for you try a 2.2uf or a 1uf & .68uf paralleled.
I'd also put a 220p across the V4-1 82k resistor into the LTPI. You can search for "enhance cap" to see an explanation for this.
with respect, Tubenit
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Thanks, will look at those changes. Appreciate the help!