Tone stack issues

[Bruce / Mission Amps]

How can there be any feedback in that tone control? The wiper of the pot goes to ground. No signal can feed back through the pot. Or can it?

EDIT: (see image)

Right. I incorrectly assumed when breezing through the texts that someone already mentioned the FB control pot was miswired and would have no effect on the cathode as a variable FB control.
If the cathode is not bypassed and the pot was wired to allow variable FB voltage to the cathode, then it would act like a frequency dependent "gain or sensitivity" control.

[J M Fahey]

Of course, absolutely OFF topic, but ...

¿do I detect some anti-Scot prejudice here?
For one, somebody writes:

sorry for the poor schematic, I'm on my mac.

Then I see the user Mc Culloch trying to obscure the fact by choosing the alias Wakculloch
What's going on here?
Or are they trying to hide a Conspiracy to re-instate Mary Stuart into the Throne?

Hey guys, I noticed nothing, eh?

[duffy878]

¿do I detect some anti-Scot prejudice here?
For one, somebody writes:

Then I see the user Mc Culloch trying to obscure the fact by choosing the alias Wakculloch
What's going on here?
Or are they trying to hide a Conspiracy to re-instate Mary Stuart into the Throne?

Hey guys, I noticed nothing, eh?

What?



...anyways, the tone control does do something, though it is subtle... I can't really tell exactly, I just know the sound does change. What I've gathered from the last few posts is that the cathode bypass cap is what's making this tone control so constrained.

Iit would not affect the upper section of the tone control, only the lower portion connected to the cathode, as shown in the diagram, correct?

[d95err]

...anyways, the tone control does do something, though it is subtle... I can't really tell exactly, I just know the sound does change. What I've gathered from the last few posts is that the cathode bypass cap is what's making this tone control so constrained.

Iit would not affect the upper section of the tone control, only the lower portion connected to the cathode, as shown in the diagram, correct?

Yes, the upper and lower part can be considered two separate circuits.

The original schematic at Adams Amplifiers has a 5k6 cathode resistor. This makes the effect of the cathode filter much bigger. With a 1k5 cathode resistor, the maximum difference it can have on any frequency is roughly 4dB.

In any case, there will probably be noticeable effect until the tone control is at the very edge of its rotation, when the resistance between the cap and ground is per has 1k or less.

[Ned]

More on this tone control

I am very interested in the Garnet tone control. I understand that it is a low insertion loss control. I have a couple questions I hope someone will address. Is it correct that the cathode resistor needs to be a certain value for the bias of the 12ax7 for maximum gain? If so will the capacitor values of the tone control need to be calculated from there? The schematic on Adams Amplifiers site shows no bypass capacitor. From the replies I can see this is probably the main issue and if I understand correctly the Garnet tone control is a variable bybass capacitor circuit that is controlling the gain and or amount of low freq. response. Have you resolved the issues with satisfactory results for your amp? If so could you post your schematic with voltages and component values? Thanks.

[duffy878]

I am very interested in the Garnet tone control. I understand that it is a low insertion loss control. I have a couple questions I hope someone will address. Is it correct that the cathode resistor needs to be a certain value for the bias of the 12ax7 for maximum gain? If so will the capacitor values of the tone control need to be calculated from there? The schematic on Adams Amplifiers site shows no bypass capacitor. From the replies I can see this is probably the main issue and if I understand correctly the Garnet tone control is a variable bybass capacitor circuit that is controlling the gain and or amount of low freq. response. Have you resolved the issues with satisfactory results for your amp? If so could you post your schematic with voltages and component values? Thanks.

Ned, this is a work in progress. In the next few days, I will be removing the cathode bypass capacitor and increasing the cathode resistor to 5.6K as pointed out by d95err. As it stands right now, I have a 3.3uF cap and a 1.5K resistor in place and the tone control is extremely subtle.

My question now is, will increasing the cathode resistor from 1.5K to 5.6K change the headroom and where the tube starts breaking up? I imagine it will, but how.

[Groover]

My question now is, will increasing the cathode resistor from 1.5K to 5.6K change the headroom and where the tube starts breaking up? I imagine it will, but how.

My $0.02: It will raise the cathode to ground voltage quite a bit -from ~1.8 volts to ~6.7 volts assuming a typically operated 12AX7 - thus biasing the tube quite a bit colder (closer to cutoff). Actually just glancing at the ECC83 load line drawings in Merlin's book it appears it might actually bias the tube completely into cutoff.

I suppose this could be dealt with by raising the grid reference - using a voltage divider in the cathode circuit. I am not sure how this would affect the operation of the cathode bypass portion of the tone control, but I think it would still work the same way.

But I am by no means an expert.

[Steve Conner]

I've got an old Selmer Corvette practice amp that uses 5k cathode resistors in both preamp stages. I posted a partial schematic in another thread.

It reduces the stage gain and increases low-order distortion, which should fatten and warm the tone. Think the same sort of effect as putting a 12AT7 in a 12AX7 socket.

I think it sounds great through the amp's 8" speaker, and I wouldn't be surprised if they did it to compensate the shrill treble and lack of bass you'd expect from an 8". I'm not sure how it would work out in a higher-powered amp.

[duffy878]

...hm, interesting Groover. Maybe I'll just start by removing the bypass cap and seeing how the tone control functions with the 1.5K.

Steve, can you post a link to that Selmer schematic, or the thread its in?

Both of you seem to be contradicting each other... If it is cold biasing, according to Merlin's book thats more towards a metal amp. I don't want that. Fat and warm sounds lovely. Anyone else care to clear this up?

[d95err]

My $0.02: It will raise the cathode to ground voltage quite a bit -from ~1.8 volts to ~6.7 volts assuming a typically operated 12AX7 - thus biasing the tube quite a bit colder (closer to cutoff). Actually just glancing at the ECC83 load line drawings in Merlin's book it appears it might actually bias the tube completely into cutoff.

5k6 is not unusually high for a 12AX7 cathode resistor. For instance, the JCM800 uses a 10k cathode resistor and the Soldano SLO 39k. The unbypassed gain will be significantly lower than when using the typical 1k5 value though.

[Steve Conner]

Here we go.
http://music-electronics-forum.com/t17670/

I didn't include the tremolo because I'm not using it any more. If you want to see a full schematic of the Corvette, there are a few at the Vintage Hofner site.

[duffy878]

Thats a cool little schematic Steve, thanks.

I think I'll have some time after work today to tinker.

[wyomingrocks]

thank you

Thanks for the link Tubeswell

[duffy878]

alright guys, so it's been quite a while....

...but I finally got around to making the parts changes. I removed the cathode bypass cap in the first triode stage and changed the cathode resistor from a 1.5K to a 4.7K (its the closest I had) and now things are acting a bit strange.

My amp 'farts' if I flip the bright switch or crank the volume.

There is a significant gain increase just at the very end of the tone control.

I'm guessing the tube is biased wrong now?

Here are my plate and cathode voltages:
PIN 1 - 273VDC
PIN 3 - 2.61VDC (no bypass cap and a resistor change)
PIN 6 - 226VDC
PIN 8 - 1.64VDC

Any ideas?