What Tubes Can I Use?

[Leo_Gnardo]

It was mentioned I could put a cap on the MV to lessen the bass (which it sounds like may be causing the bad tone with the MV) ,but I never got details for how to do that with the MV I have.

As Helmholtz wrote: Caps on the MV don't lessen the bass but increase high treble at lower MV setting.

Treble, yes and extending into midrange depending on how large a cap is used. The reason I suggested this is "there's too much bass." By allowing more highs/mids through MV is dialed down, the guitar signal retains more presence and doesn't become too wooly.

WHAT VALUE TO USE? All depends on what the user wants to hear, try 47 pF up to .005 uF. Smaller values allow a little high end sizzle through. Larger values, all the way down into the midrange. Somewhere, you might find your sweet spot. Only you can know what works for your ears, by experimenting with your amp.

HOW TO APPLY: between hot and wiper of the MV pot. If PPIMV, BOTH POTS.

[gtrplayr1976]

As Helmholtz wrote: Caps on the MV don't lessen the bass but increase high treble at lower MV setting.

Treble, yes and extending into midrange depending on how large a cap is used. The reason I suggested this is "there's too much bass." By allowing more highs/mids through MV is dialed down, the guitar signal retains more presence and doesn't become too wooly.

WHAT VALUE TO USE? All depends on what the user wants to hear, try 47 pF up to .005 uF. Smaller values allow a little high end sizzle through. Larger values, all the way down into the midrange. Somewhere, you might find your sweet spot. Only you can know what works for your ears, by experimenting with your amp.

HOW TO APPLY: between hot and wiper of the MV pot. If PPIMV, BOTH POTS.

Ok ,I'll ask a stupid question here ,what would be the hot side? Leads to the board ,or from the tubes?

[The Dude]

The hot side is the side that's not ground. That said, we have yet to see a schematic and don't know what you built. Depending on the type of MV, there may not be a ground side.

[gtrplayr1976]

The hot side is the side that's not ground. That said, we have yet to see a schematic and don't know what you built. Depending on the type of MV, there may not be a ground side.

Really didn't use a schematic. I used a layout from a kit even though this wasn't a kit. I posted that above ,and it shows the MV.

[Leo_Gnardo]

Ok ,I'll ask a stupid question here ,what would be the hot side? Leads to the board ,or from the tubes?

On your layout, there's a thick black line that connects two of the pot's terminals to the circuit board, that's your AC ground. Note, it's not a zero volt chassis ground, it connects to your bias supply. The filter cap in your bias supply passes any signal to ground, that's why we call it an "AC ground." As you dial the pot down in volume the wipers (middle terminals) move towards that AC ground, away from the signal "hot" terminals at the other end. Caps go between hot and wiper, one on each pot.

For starters, I'd say try 470 pF. I'm sure you'll hear a difference. If that's too bright for you, sub in smaller caps. Not bright enough? Go larger.

No 470's in stock? Anything in that range will do, say 220 pF or .001 uF. Try it & you're bound to retain some clarity in the signal as you dial down the MV.

[gtrplayr1976]

On your layout, there's a thick black line that connects two of the pot's terminals to the circuit board, that's your AC ground. Note, it's not a zero volt chassis ground, it connects to your bias supply. The filter cap in your bias supply passes any signal to ground, that's why we call it an "AC ground." As you dial the pot down in volume the wipers (middle terminals) move towards that AC ground, away from the signal "hot" terminals at the other end. Caps go between hot and wiper, one on each pot.

For starters, I'd say try 470 pF. I'm sure you'll hear a difference. If that's too bright for you, sub in smaller caps. Not bright enough? Go larger.

No 470's in stock? Anything in that range will do, say 220 pF or .001 uF. Try it & you're bound to retain some clarity in the signal as you dial down the MV.

Thank you sir. I have plenty of caps. Does it matter what type they are?

[eschertron]

For starters, I'd say try 470 pF. I'm sure you'll hear a difference. If that's too bright for you, sub in smaller caps. Not bright enough? Go larger.

No 470's in stock? Anything in that range will do, say 220 pF or .001 uF. Try it & you're bound to retain some clarity in the signal as you dial down the MV.

And I'll add that the effect you want is wherever the MV gets set for the bulk of the playing. At higher MV settings, you increase bass, and treble.. not so much. It's already bypassing the attenuator.

[Leo_Gnardo]

Thank you sir. I have plenty of caps. Does it matter what type they are?

Typically smaller values will be disc ceramic, as you go up in range discs are still OK but you can use any film cap. No super cheap low voltage caps, I'd say 200VDC rated at minimum. It's only signal on them in this application, but that signal could be fairly large as it's driving the output tube grids, approaching 100V p-p if you're whacking that amp hard.

[gtrplayr1976]

Typically smaller values will be disc ceramic, as you go up in range discs are still OK but you can use any film cap. No super cheap low voltage caps, I'd say 200VDC rated at minimum. It's only signal on them in this application, but that signal could be fairly large as it's driving the output tube grids, approaching 100V p-p if you're whacking that amp hard.

I'm sure I have some Mica ,and probably ceramic disc as well. Not sure I have any film caps that small except maybe the .001.

[pdf64]

Agree but the problem is going the other way, as Justin outlined above. For example the era when Marshall dropped EL34's due to reliability issues and had stickers on their amps that said 'equipped with 5881's'. Those were the WXT's like Justin said, and had no problem meeting the 6L6GC requirements. A vintage or re-ish proper 5881 is probably not going to do well in such a circuit. Same as all the Fender amps that came stock with the Sovtek 5881/6L6, the 5881 moniker was being used in a non-traditional manner.

I thought it may be worth going into detail for the benefit of the OP, to save him all that nonsense of tweaking and calculating to achieve idling at 70% for each tube type, ie the idle current appropriate for a 23W 6L6 type tube is also completely valid for a 26W or 30W 6L6 type tube, there's no technical reason to idle the higher rated tubes any hotter.

[gtrplayr1976]

So I went in the middle ,used 330pf ceramic disc. Definitely helped. I may try some 6L6GCs ,and KT77s to see how big a difference there is. Thanks for all the advice. I will be doing more experimenting as I go.

[Leo_Gnardo]

So I went in the middle ,used 330pf ceramic disc. Definitely helped.

Yaaay!

I may try some 6L6GCs ,and KT77s to see how big a difference there is. Thanks for all the advice. I will be doing more experimenting as I go.

OK, keep us posted.

[gtrplayr1976]

Yaaay!



OK, keep us posted.

Will do.

[pdf64]

Just to note that I too find the JTM45 circuit way too bassy, but dislike the response from its tone stack if the treble is turned much above halfway (linear pot). However, slightly reduced value coupling caps between the input stages and their vol controls (10nF normal, 3n3F bright) seems to reduce the extreme bass extension and clean up subsonic hash when it's overdriven. And a bigger bright cap, at least 220pF, on the channel vol controls. I didn't like the treble peaking cap across the mixer resistor that some version use.

But the big saving grace with the circuit is the extreme degree of treble boost available when the presence is turned up full, which is where I leave it, permanently. And I now prefer my JTM45 type amp over even my old Vox AC30.
Though with mine, compared to the OP's, I think that KT66 and the original 'shared+individual' screen grid resistor arrangement are key to that preference.

Pertinent to this thread is that I noticed the OP's circuit uses the original 27k/5k NFB ratio, but taken from the 8 ohm tap, rather than the 16ohm. So the degree of NFB, and hence presence boost available, is reduced somewhat (~30%?).
But more significant than that is the use of a dual track master volume between the LTP phase splitter and power tubes, as I understand things?
As that is turned down, the power amp's loop gain will reduce, and along with that the available degree of boost from the presence control.
Hence I feel that given the issue (amp way too bassy), such a master volume is unsuitable and perhaps may be part of the problem, as it disables a very necessary lifeline, that in the amp's stock form is essential to brighten up the amp's response.

And all this stuff with putting bright caps on to that master vol is just akin to trying to polish a turd of a mod
If a master volume is wanted, then start with a proven good design that uses one (eg 2204), rather than hammering a square peg into a round hole by trying to get good results from fitting a master vol in a classic non master vol circuit.
But that's only my opinion, whatever works for the OP is good.

[gtrplayr1976]

Thank you pdf64. I'm always open to improvement. I'm not a "real" builder. I can do the work, but I don't really understand everything ,and how it's affected. At this point I'm not really happy with the MV operation ,so I very well could change it.

I also noticed last night while looking at the layout I used ,and some pictures I had taken ,that I may have left out a connection point in the amp. The amp works with little to no noise ,so I'm not sure exactly how this would affect the amp? That cap goes to ground ,and the connection between that resistor ,and cap may be missing. I'm going to look to be sure I didn't come back ,and add that at some point. The part in the circled are is what I may have left out.