Hi all,
after years of Big Talk I finally built the guitar amp I'd been thinking about for years. Still needs a few tweaks, but it's already gig-able and working/sounding really sweet.
Here's the question: I notice to my surprise that the tube which seem most sensitive to direct physical disturbance is not the (very very) high gain input pentode nor one of the other gain-providing stages, but the one hosting the cathodyne phase splitter and another near-unity stage with the same equal plate and cathode loads. I am gathering that what makes these more microphonic is the fact that the idle cathode voltage is elevated from that of the heaters - I read this somewhere, at any rate. It's a 6SN7 and the cathodes are at roughly 85V so I'm nowhere near the limit on the datasheets (200V positive) even with maximum possible signal going through there.
I am not experiencing a particular problem - I can set the head on top of my speaker, and if it's picking up something extra microphonically, I can't necessarily hear it. If you tap that tube directly (I know I know, "don't do that") it's pretty loud. Even before you tap it you can hear the heater hum increase when your hand gets near to it.
Here's the question for real: Is there any trick for quieting it down I should know? I'd just feel better about my creation if it didn't act like this.
after years of Big Talk I finally built the guitar amp I'd been thinking about for years. Still needs a few tweaks, but it's already gig-able and working/sounding really sweet.
Here's the question: I notice to my surprise that the tube which seem most sensitive to direct physical disturbance is not the (very very) high gain input pentode nor one of the other gain-providing stages, but the one hosting the cathodyne phase splitter and another near-unity stage with the same equal plate and cathode loads. I am gathering that what makes these more microphonic is the fact that the idle cathode voltage is elevated from that of the heaters - I read this somewhere, at any rate. It's a 6SN7 and the cathodes are at roughly 85V so I'm nowhere near the limit on the datasheets (200V positive) even with maximum possible signal going through there.
I am not experiencing a particular problem - I can set the head on top of my speaker, and if it's picking up something extra microphonically, I can't necessarily hear it. If you tap that tube directly (I know I know, "don't do that") it's pretty loud. Even before you tap it you can hear the heater hum increase when your hand gets near to it.
Here's the question for real: Is there any trick for quieting it down I should know? I'd just feel better about my creation if it didn't act like this.
Schematic, perhaps? I saw a schematic somewhere on line, where a heater isolation transformer was used, to float the heater above ground. Not so elegant expense wise, but I assume it worked. There was even a Hammond model number for the transformer they used.
Unfortunately, that was seen on my garage PC, as I was looking over transformer coupled tube circuits and apparently Gogl thought that might be what i was looking for. I'm on my laptop now.
Unfortunately, that was seen on my garage PC, as I was looking over transformer coupled tube circuits and apparently Gogl thought that might be what i was looking for. I'm on my laptop now.
Don’t have the schematic handy right now, but I do have a heater winding on the transformer which is just being used for the power indicator lamp. When I first wired it up I had all the heaters split between the two available 6.3V secondaries but I found the voltages way too high, more like 7.5V. My solution was to put all the heaters on one secondary, but maybe I should try something. I’ve got a whole list of things I’d do differently “next time”.
well, just as a first experiment I decided to add a 10Kohm grid stopper to the first half of the 6SN7. The circuit in question is one I was asking about a couple years ago in this thread. Its then loaded by the other grid of the tube, a cathodyne phase splitter. Both of these stages are set up very similar to each other, each being loaded by 20Kohm resistors in both the plate and cathode.
After playing around with it for awhile, I feel like it's less sensitive than before. I'll keep playing and listening.
I found this little tidbit from the Valve Wizard which will probably be the next thing I try:
After playing around with it for awhile, I feel like it's less sensitive than before. I'll keep playing and listening.
I found this little tidbit from the Valve Wizard which will probably be the next thing I try:
I had the thought that I'd use a separate winding and elevate it just for this one tube, but maybe I'd try elevating everyone. It would sure be easier.
being unsatisfied with the grid stopper alone (worth doing probably but insufficient), I did float all the heaters up to about 60Vdc. I made a divider from the B+ with a 240K and a 47K.
What I’m realizing though is that the heaters will be near ground until I flick the standby switch. Then they’ll quickly bounce up to 60V. I’m wondering if this should be a concern?
I realize the standby switch is controversial by itself. The main value I see in it is for just in case I forget to plug in my speaker cab. But for now I’ve just been leaving it in the on position.
What I’m realizing though is that the heaters will be near ground until I flick the standby switch. Then they’ll quickly bounce up to 60V. I’m wondering if this should be a concern?
I realize the standby switch is controversial by itself. The main value I see in it is for just in case I forget to plug in my speaker cab. But for now I’ve just been leaving it in the on position.
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