NOS Astron mustard caps in 5f6a Bassman

[Rod64]

Hi,
I have a 5f6a kit amp that has orange drop capacitors. No issues. Recently, I found a set of NOS 0.08mF 200VDC Astron mustard caps. I know that these were not as reliable as the Blue Molded. I tested them and the all come out fine. I was thinking of swapping them for the two 0.1 mF coupling caps and the one in the tone stack. Has anyone tried this with old caps? Worth a try or waste of time?
Thanks

[maxkracht]

Low voltage cap test doesn't necessarily translate to what happens with high voltage. Double check for leaks with full voltage if you haven't already. I wouldn't put them right before the power tubes but as a presence cap would be fine, tone stack might be fine. What is worth it to you is up to you. Yellow astrons can be hit and miss with reliability, but nos might be fine if they were stored properly.

[sluckey]

I found a set of NOS 0.08mF 200VDC Astron mustard caps... I was thinking of swapping them for the two 0.1 mF coupling caps and the one in the tone stack.

Don't do it. 200V rating is too low.

[Rod64]

That was also an issue I think the voltage capacity is usually 400-500V. Thanks for heads up.

[maxkracht]

Tone stack should be under 200v, but there won't be much margin.

[bepone]

at first powering on without standby, everything is on max voltage.

200V cap can be used only in presence circuit and PI second grid which is grounded for AC via C

[sluckey]

at first powering on without standby, everything is on max voltage.

Not so in this case. The cathode of that cathode follower will be at zero volts and slowly rise as the tube heats up and begins drawing current.

[bepone]

at first powering on without standby, everything is on max voltage.

Not so in this case. The cathode of that cathode follower will be at zero volts and slowly rise as the tube heats up and begins drawing current.

where is grid of CF connected ? directly coupled to anode?
and what is the voltage there if anode current is zero?
Ub+ max or not?

[sluckey]

at first powering on without standby, everything is on max voltage.

Not so in this case. The cathode of that cathode follower will be at zero volts and slowly rise as the tube heats up and begins drawing current.

where is grid of CF connected ? directly coupled to anode?
and what is the voltage there if anode current is zero?
Ub+ max or not?

Doesn't matter what's on the grid. The TS caps are connected to the CATHODE and until the heaters warm up the cathode, no current will pass through the tube or cathode resistor. No current = no voltage.

Furthermore, it takes about 15 seconds for that GZ34 to heat up enough to allow any B+ voltage in the amp.

[bepone]

Doesn't matter what's on the grid. The TS caps are connected to the CATHODE and until the heaters warm up the cathode, no current will pass through the tube or cathode resistor. No current = no voltage.

Furthermore, it takes about 15 seconds for that GZ34 to heat up enough to allow any B+ voltage in the amp.

so grid will be on 400+V and cathode to 0? how the tube will survive this then? Ugk = +400V you say until heat up.

try to check and let me know.

[bepone]

also for the people thinking that voltage on cathode follower is less than 200V, it is not always, it is from 180-220VDC in real operation, enough to not to use old 200V caps there.

[sluckey]

try to check and let me know.

If you want to know, try it yourself. Just disconnect one of the filament wires on the CF.

[maxkracht]

I can check. In no way did I mean to imply that using 200v caps would be a GOOD idea, but I still think you might be able to get away with it. I would hope that 200 is the working voltage and there is some surge capability, but old caps are old caps...

[martin manning]

~200V is the expected DC voltage at the cathode of the CF, with signal peaks above that. I would go with 400V for the tone stack caps, as specified on the schematic ;^)
The g-k voltage on the CF tube at start-up is a separate issue, but it's easy to protect it with a rectifier diode and a resistor as recommended by Blencowe.

[maxkracht]

My wall voltage isn’t that stable today so forgive a bit of variation. 



Cathode is normally at 185-190v ramps up to 230v briefly during power up before settling back down.
Grid gets full 450-475v then down to 190 after everything is warm, unless you add a neon from grid to cathode, then it stays under 240v. 
Grid to cathode voltage surged to 475 until tubes heat up. With a neon, Grid to cathode was 75v until heat up.

Neon is a good idea, though I once noticed some weird distortion when using a smaller neon. Didn't have time to investigate that one instance, but I have stuck with larger versions since noticing that and haven't had issues.