Sure i can do that.
for what it's worth, with 220r strapped across the input, I only have squeal when the gain 2 knob is above 80%.
You have unseen talents.
Next it might be good to bypass the plate resistors with a small capacitor to find which stage is the most offending one. Then determine your best values.
https://www.ampbooks.com/mobile/amplifier-calculators/plate-bypass-capacitor/
Just an odd thought, could you put some resistance in series with the speaker to get the reflected impedance closer to what the tube wants?
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"What the tube wants" is sort of unknown? The ssm schematic says 10k. The original squirrel monkey schematic said 8k, but the designer later remeasured his transformer and determined that he was at about 3.5k.
I am actually using a stancor universal opt right now at about 4k. I can configure it for higher impedance if that might help.
Bypassing plate resistors with 1000pf hasn't helped. I will look for higher values in my parts bins later, i have family stuff to attend to now.
I know that technically the cap only 'sees' the voltage drop across the resistor but most of my caps are rated under 100v
.:Sent by pneumatic tubes
I am actually using a stancor universal opt right now at about 4k. I can configure it for higher impedance if that might help.
Bypassing plate resistors with 1000pf hasn't helped. I will look for higher values in my parts bins later, i have family stuff to attend to now.
I know that technically the cap only 'sees' the voltage drop across the resistor but most of my caps are rated under 100v
.:Sent by pneumatic tubes
47nf across R9 does silence the squeal. There's some motorboating with the master volume near max in this configuration. Probably silences things i want to hear, too.
So, assuming either of my scopes still work (both older HP gear) and i can find the probes, what should i probe at? For giggles, I also have a Wavetek model 171 synthesizer and function generator.
If the 2nd section (6af11 triode 1) really is the bastard, how do i bend it to my will?
So, assuming either of my scopes still work (both older HP gear) and i can find the probes, what should i probe at? For giggles, I also have a Wavetek model 171 synthesizer and function generator.
If the 2nd section (6af11 triode 1) really is the bastard, how do i bend it to my will?
Nice thing about scopes, they are pretty tough and you can feel your way around. A X10 probe will do you good, I picked up some cheap ones and they are fine. Should be some tutorials on how to use a scope but for this kind of stuff just getting the sync, gain and frequency right is all you need. We really only care to see the signal, in this case the feedback. Falling into something tends to cascade along in your life.
You have part of your answer already.
The motorboating sounds more like a power supply cap. If that does not help as a 'really' WAG try reversing your primary leads on the transformer. Had an amp where I was getting squeals and it was the transformer feeding back to the input even with them far away from each other.
Yeah, I responded to an ad for a free scope, just had to be willing to drive out to a remote-ish development to get it. I show up in my 2-door VW GTI and they tell me there's more and i have to take all of it.
Seems their friend had been an amateur amp designer of some reasonable skill who had left behind his scrounged lab when he got a high paying gig on the east coast, with instructions to sell it, and had never been heard from again.
They'd grown tired of trying to sell it after something like 7 years and just wanted rid of it.
It all barely fit in my car. Some of the gear is mounted in a 19" rack that is about 5 feet tall. I have multiple copies of the manual for some of it, and should list those on ebay. There's a DIY bench supply with a transformer the size of a toaster oven and capacitors with a volume in excess of 1L (along with several smaller transformers and sets of caps) that i am mildly terrified of firing up, except it includes a giant variac too so i am sure i could find a safe way to do it.
So, I have an HP clamp milliammeter from the 60's, and a Telequipment transistor curve tracer, and a bunch of other stuff i might never use but which has limited resale value compared to it's possible usefulness.
I'm reasonably certain that i have at least three 10x probes and that at least one of them is Agilent or similar reputable quality. Where, is more the question.
Well, it appears only when the volume and gain knobs are near max, just like the squeal.
I vaguely recall that the power supply caps that weren't bought brand new tested ok in my (modern, cheap) ESR meter.
I will try reversing the hv primary leads.
You have two reasons for feedback. You are increasing the gain and the amplified signal is adding to the unamplified signal. What can reduce this margin is when the amplified signal changes phase and is in phase with the unamplified signal. The change in phase does not have to be 180 degrees, lesser angles also increase the susceptibility for feedback. The ends of the audible spectrum are sometimes problems for tube amps, especially due to the transformer. You get capacitance and inductance that mess with the phase of your signal at both extremes. Motorboating is a common problem with IC's where the supplies have the capacitors a fair ways away. Basically the capacitors are no longer acting as a short to the frequencies in question. Solving the problem is with some insight and trial and error. Only some problems will show up in an amp, hopefully the process of elimination will get you where you want to go.
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