ampeg tone control

[ernie_jr]

Happy holidays to all.
I have question about the ampeg tone controls on the b18, b18, b25, etc. It calls for a 0.01 and 0.001 across the bass pot. I am out of 0.001 caps but have plenty of 0.0015 caps. will these work?
Also, what voltage do I need. I did find some 0.001 100 volt caps.
thanks,
Ernie

[TubeGeek]

0.001 and 0.0015 uF are so close I'd say go for it.

Not sure about the voltage, measure the voltage on the cap and see what it is and then decide.

[DummyLoad]

DC is blocked by the coupling cap... so whatever the Vpk swing is - i'd say at least 250V (> 2/3 of 345V B+) of that node to be on the safe side. i don't think 100V part will last long.

--pete

[stratele52]

Caps value are same , voltage to low . Need 400 volts minimum

[sluckey]

Those tone control caps do not see any DC voltage and the only AC voltage present is the approximately 1 VAC signal voltage. I would not hesitate to use even a 50v cap for the tone control caps.

The coupling cap from the previous stage plate does need to be rated higher. I'd use 500v or 630v.

[stratele52]

I do not think it's a good idea to use capacitors at low voltage (100 V) in a tubes amplifier. Moreover, no serious manufacturer does to my knowledge.
Why not take the time to order the right parts and keep a high standard of quality in your work?

[sluckey]

I do not think it's a good idea to use capacitors at low voltage (100 V) in a tubes amplifier. Moreover, no serious manufacturer does to my knowledge.
Why not take the time to order the right parts and keep a high standard of quality in your work?

Well, I think Fender takes their product seriously. Fender has used caps rated for only 25VDC in their tube amps for about 60 years. And the cap in the AB763 NFB circuit is rated 200V. There is nothing substandard about using "voltage appropriate" caps.

[stratele52]

I do not think it's a good idea to use capacitors at low voltage (100 V) in a tubes amplifier. Moreover, no serious manufacturer does to my knowledge.
Why not take the time to order the right parts and keep a high standard of quality in your work?

Well, I think Fender takes their product seriously. Fender has used caps rated for only 25VDC in their tube amps for about 60 years. And the cap in the AB763 NFB circuit is rated 200V. There is nothing substandard about using "voltage appropriate" caps.

You mean bypass electrolytic cap on 12XX7 tubes .

I talk about most of the coupling caps . 200 V caps in NFB is twice than 100 volts caps . 

My opinion is to put same or better than original parts . If I replace a 400 volts caps , I won't put a 200 volts  caps .

[jjasilli]

Those tone control caps do not see any DC voltage and the only AC voltage present is the approximately 1 VAC signal voltage. I would not hesitate to use even a 50v cap for the tone control caps.  The coupling cap from the previous stage plate does need to be rated higher. I'd use 500v or 630v.

Ditto.  This Ampeg tonestack is fully protected from DC by the coupling (blocking) cap between it and the driving stage.  Hence, the caps within this Ampeg tonestack see only a small AC signal as Sluckey says.

In TMV style tonestacks there usually is no coupling (blocking) cap between it and the driving stage.  Instead signal and full DC voltage branches-off, one leg through the treble cap; the other leg through the mid & bass caps.  All of these tone caps need to be rated for the full DC operating voltage. But if you add a blocking cap rated for full DC voltage before the tonestack, you then could use low voltage caps in the TMV tonestack.  Per the rule of 10's, the value of such blocking cap should be at lease 10X higher than the TMV's tone caps, so as not to otherwise effect the circuitry.

 

[sluckey]

You mean bypass electrolytic cap on 12XX7 tubes .

Yes. 25V was appropriate and very available.

I talk about most of the coupling caps .

You never specified you were talking about coupling caps. And you suggested that it would not be "a high standard of quality in your work" to use a low voltage cap, even though the voltage present in the circuit is verrrrry low. That's just wrong. Besides, ernie_jr never mentioned coupling caps. He was talking specifically about the tone caps in a particular amp. Ampeg was fond of using a potted package that contained those caps as well as the resistors in that tone network. Made a very neat installation directly on the pots.

My opinion is to put same or better than original parts . If I replace a 400 volts caps , I won't put a 200 volts  caps .

I agree whole heartedly. But in this case we don't know what the voltage rating of those caps actually is. So it's up to us to examine the circuit and use voltage appropriate caps. Sure, you can use higher voltage caps. And if that's what I had in stock that's what I'd do too. But there certainly is nothing wrong or substandard about using a 100V cap in that particular circuit.
 

 
 
 

[ernie_jr]

Thanks to all.
Thanks Sluckey, I think I will use to 100volt caps as I have a boat load of them. I am building a SUNN/Ampeg clone.
one channel SUNN 200s, 2nd channel Ampeg B25. PI and power section from SUNN with KT 120's running about 560 volts on the plates.
Will keep updates posted.
Ernie