Lower than usual gain on a modded TAD JTM-45 kit

[VintageCharlie]

Hi guys,

i've run a full circle after building a jtm45/100 replica and exploring many other great amps. I realised that the JTM-50 might probably be THE sound i was looking for many years, so i thought i'd buy a TAD JTM45 that had been put together by TAD and mod it into a JTM-50.

I received the amp and i think it sounds great despite the few alterations it has what i was not aware of before buying. It sounds darker, bolder and louder than what i remember a JTM45 to be, which i owned a long time ago (it actually sounds EXACTLY more of like what i was hoping to find in a JTM-50). BUT it also does have less gain than for example my 45/100, which should not be the case. Coincidentally it seems like some of the alterations already bring it quite close to a JTM-50 spec, more so than a JTM-45.

So i checked the premap tubes and swapped them with another set - no change in gain. So it's not the tubes.

The amp is a JTM45 Kit with these alterations:

- a 50w JMP style OT (running KT66 on 8ohm tap, which is also wired to the 8 ohm tap of the OT)
- The PT LOOKS like the JTM-45 PT from TAD, so it is probably putting out 345v instead of 325v - need to measure that. I assume this would give a bit more headroom, but i do not expect it to be a big deal, especially as it is still running KT66 tubes.
- 32uF filtering throughout (2x dual 32uF caps) (this might make the amp a bit cleaner sounding, correct?)
- split cathode on V1
- .68uF/2k7 bypass cap (which from what i know should add gain)
- 500pF mica also on the normal channel mixer resistor
- Bias supply dropping resistor is only 100k
- 27k resistor between the 8uF bias caps (which usually is 56k)
- 10k voltage drop resistor instead of the typical 8k2
- NFB resistor is standard 27K
- there are 5K6 resistors on the output tubes
- one of the .1uF caps has been replaced with a smaller sized .1uF cap
- it has PPIMV installed and some alterations have been done on the board - the 220k resistors have not been placed on the board but have been moved directly onto the dual pot and additional 270k resistors have been installed on the pot. (i am wondering, if this is sonmething that might limit how hard the output stage is being hit, as i have not seen it done this way and i do not understand what has been done here to the PPIMV. Please see the attached pic for reference.
- there is a 47nF cab going from the Rectifier socket to gound - no idea if that is standard for a JTM-45 or JTM-50.

The amp seems to be done well - e.g. the inputs are screened cables, the ppimv implementation looks clever to a noob like me. The 68k input resistors have been added directly to the jacks to shorten the signal travel length - so i feel like the person who put this together knows what he/she was doing. Some changes on the other hand seem to have been done by someone doing it in a quick and dirty way (e.g. the smaller sized .1uF cap in the PI, the 100k resistor next to the 820r)

Please let me know, if in your view some of these things might reduce the gain of the amp.

Otherwise i have no complaints - it sounds very much in the Hendrix direction with a strat and nice classic rock with humbuckers. Just a low on the gain - cranked on the bright channel with a strat it sounds like a semi/clean Hendrixy type of "clean tone", which i would expect at about 7 on the dial.

Next steps would be to change to EL34s, change to shared cathode on v1, remove the 500pF on the normal channel mixer resistor and maybe remove the PPIMV, as i have no need for it and i suspect it might be doing something i do not want to be done.

Thanks for your input and any hints!

[sluckey]

it has PPIMV installed and some alterations have been done on the board - the 220k resistors have not been placed on the board but have been moved directly onto the dual pot and additional 270k resistors have been installed on the pot.

There is no 220K on the MV. The 2.2M resistors are safety resistors that keep bias voltage on the output tubes in the event the pot wiper fails. The 270K CC resistors are across the pot to lower the resistance of the pot, ie, make a 500K pot look like a 175K pot or a 250K pot look like a 130K pot. All of this should be fine.

[JD0x0]

What's going on with those two resistors to the right of the treble cap? AFAIK there should only be one resistor, the cathode follower's cathode resistor.

[pdf64]

What's going on with those two resistors to the right of the treble cap? AFAIK there should only be one resistor, the cathode follower's cathode resistor.

Isn’t it the 820ohm cathode resistor of the preceding, 2nd stage?
Actually, bypassing that should give a little more gain.

[JD0x0]

What's going on with those two resistors to the right of the treble cap? AFAIK there should only be one resistor, the cathode follower's cathode resistor.

Isn’t it the 820ohm cathode resistor of the preceding, 2nd stage?
Actually, bypassing that should give a little more gain.

Ah I see it now. I couldn't tell where the under board wires were going and I wasn't able to read all the color bands, so it came out to a weird value, but that makes sense, now that you mention it.

Bypassing that cathode with a cap should give a significant amount of gain and increased clipping, but if the OP doesnt have that stage bypassed on their other amp, and the amp lacks gain compared to their other amp, I'd say something else might be at play.

[Stevem]

Someone correct me if I am wrong, but the EL34s used in a jtm50 have near twice as much u as a kt66 and your for sure going to notice that difference in the feel!

[Stevem]

Someone correct me if I am wrong, but the EL34s used in a jtm50 have near twice as much u as a kt66 and your for sure going to notice that difference in the feel between two amps ,all things being Apples to Apples!

Also adding another bypass cap is going to change the tonal structure of the sound also, and this needs to be auditioned for being a help or harm to the cause!

[VintageCharlie]

Someone correct me if I am wrong, but the EL34s used in a jtm50 have near twice as much u as a kt66 and your for sure going to notice that difference in the feel between two amps ,all things being Apples to Apples!

Also adding another bypass cap is going to change the tonal structure of the sound also, and this needs to be auditioned for being a help or harm to the cause!

I tried to mismatch the secondaries of the OT to compensate for that and there was a tonal change, but not a difference in gain worth mentioning.
The bypass cap surely tightened up the amp and removed some flub (probable leaving also the impression of less gain due to decreased bass).

What's going on with those two resistors to the right of the treble cap? AFAIK there should only be one resistor, the cathode follower's cathode resistor.

Isn’t it the 820ohm cathode resistor of the preceding, 2nd stage?
Actually, bypassing that should give a little more gain.

Ah I see it now. I couldn't tell where the under board wires were going and I wasn't able to read all the color bands, so it came out to a weird value, but that makes sense, now that you mention it.

Bypassing that cathode with a cap should give a significant amount of gain and increased clipping, but if the OP doesnt have that stage bypassed on their other amp, and the amp lacks gain compared to their other amp, I'd say something else might be at play.

There is an added treble resistor on the normal channels mixer - will have to check, if that also has an effect on gain of the bright channel, as it is not a stock Marshall value there.
What i did not realize is that the 2K7 resistor along the .68uF bypass cap relativates the added gain of the cap. So i just switched to shared cathode with the 820r/250uF and it immediately moved the amp where i expected a JTM45 to be tone and gain wise. Maybe a bit tamer than my 45/100, but the difference is much smaller now.

it has PPIMV installed and some alterations have been done on the board - the 220k resistors have not been placed on the board but have been moved directly onto the dual pot and additional 270k resistors have been installed on the pot.

There is no 220K on the MV. The 2.2M resistors are safety resistors that keep bias voltage on the output tubes in the event the pot wiper fails. The 270K CC resistors are across the pot to lower the resistance of the pot, ie, make a 500K pot look like a 175K pot or a 250K pot look like a 130K pot. All of this should be fine.

Thanks for clearing that up! I misread the 2M2 resistors.

[VintageCharlie]

Thanks for all the input, guys! That helped a lot.

I had wrongly assumed that the split cathode arrangement with the separate .68uF/2K7 on the cathode would add gain, but did not realize that the 2K7 actually attenuates quite a bit in addition of the .68 theoretically adding gain. So it made the amp much tighter, tidier and much less JTM-45 sounding - really a huge difference. So thats pretty much clear now.

I will look into some things that might give a bit of a gain increase while using the split cathode arrangement - i really liked the tonal changes it made.

So next steps

- check, if the additional treble cap on the normal channel 270K mixer has any noticeable effect on the bright channel (it sure does make the normal channel much more useable than stock)
- Try increasing NFB resistor for more gain
- Change mixer resistors to 470K

[JD0x0]

FWIW, if you had fully bypassed the cathode with both resistor values, there shouldn't be much difference in gain between a 2k7 and the (shared) 820R, but since a .68uF doesnt fully bypass all frequencies, you'll have a bit less gain in the low frequencies using that value. This is easily compensated for by increasing the cathode bypass cap value, if you want to keep the load line of the 2k7, but have more gain in the low frequencies, like the original setup.

A .68uF will increase gain at treble and mid frequencies vs no cathode bypass cap.
A 25uF-250uF will increase gain at virtually all frequencies vs no cathode bypass cap.

The cathode resistor changes the knee of the roll off point and affects the gain of the unbypassed frequencies. As you bias colder, the stage will have more negative feedback on the frequencies unbypassed by the cathode cap, which reduces the gain a bit. So, had you kept the 25uF-250uF bypass cap, you likely would've not noticed any gain loss with the higher value cathode resistor. With a 100k plate resistor, both setups would've had a gain around 59.26 (35.46 dB) from 10khz to 82hz with either cathode resistor, if a 250uF cap were used.

If you want the split cathode arrangement with the .68uF/2k7 values, but an option for more grunt, you can cascade the channels. The reduced bass of the .68uF will help clean up bass flub for the higher gain tones. You could also easily add a switch to parallel more capacitance to make for a sort of 'tight/full' switch, which isn't that uncommon to do in amps.

[VintageCharlie]

FWIW, if you had fully bypassed the cathode with both resistor values, there shouldn't be much difference in gain between a 2k7 and the (shared) 820R, but since a .68uF doesnt fully bypass all frequencies, you'll have a bit less gain in the low frequencies using that value. This is easily compensated for by increasing the cathode bypass cap value, if you want to keep the load line of the 2k7, but have more gain in the low frequencies, like the original setup.

A .68uF will increase gain at treble and mid frequencies vs no cathode bypass cap.
A 25uF-250uF will increase gain at virtually all frequencies vs no cathode bypass cap.

The cathode resistor changes the knee of the roll off point and affects the gain of the unbypassed frequencies. As you bias colder, the stage will have more negative feedback on the frequencies unbypassed by the cathode cap, which reduces the gain a bit. So, had you kept the 25uF-250uF bypass cap, you likely would've not noticed any gain loss with the higher value cathode resistor. With a 100k plate resistor, both setups would've had a gain around 59.26 (35.46 dB) from 10khz to 82hz with either cathode resistor, if a 250uF cap were used.

If you want the split cathode arrangement with the .68uF/2k7 values, but an option for more grunt, you can cascade the channels. The reduced bass of the .68uF will help clean up bass flub for the higher gain tones. You could also easily add a switch to parallel more capacitance to make for a sort of 'tight/full' switch, which isn't that uncommon to do in amps.

Thanks for the explanation and ideas! Today i played around with the typical Marshall value variations on the amp - tried .68uF cap on V2 (certainly did not like that one), 500pF/33k tone stack (i think with the 33k came a weird mid hump that i did not like with humbuckers), 470K mixers (very subte difference which i would not hear, if would have not recorded clips of the same loop before and after) - the 500pF/33k tone stack gave more gain and a brighter tone. Though the 470k mixers are said to make the tone a smidge darker, in my a/b clips i have the impression that the 470k gave a bit more bite. I kept the 500pF cap across mixers, which actually should be decreased to around 390pF, if one wants it to be as effective as the 500pF is with 270K mixers for brightening the tone.

All in all i have the feeling that the JTM circuit profits most from a bass reduction early in the circuit. The V1 cathode caps and resistors are a great place to shape the tone. I am not 100% certain yet, but i feel like, if i want more gain, i can get a better tone just boosting the amp instead of trying to squeeze the gain from various points in the amp itself, as it seems like that always comes at expense of a harsher and less organic tone (all subjective of course).

[Reeltarded]

Bypass big over the 820.. uhh 470 mixers, 330p treble. 68-82k slope.. 100p bright.

The amp loses all that mmmmphy mid clutter. Humbuckers have a big mouth and single coils sound like million pound acoustics.

Secret: parallel midrange control <--- this is meaningful.. very, very meaningful. The bass and treble interact and the mid range stays where you put it and sounds fantastic wherever it was... putted.

[Littlewyan]

Having a 500pf over the normal channel mixer resistor will reduce the gain and brightness of the bright channel. It’ll be especially noticeable with the normal channel volume on 0, at that point you’ve basically got a 500pf to ground on the bright channel. The plus side is it makes the normal channel more usable, but personally if that was the goal I would just remove both caps altogether so you just have the 270K mixer resistors.

Also 470K mixers with a 500pf bypass cap will be brighter than 270k/500pf, but with a 500pf over the normal channel mixer you won’t hear much difference.