Somewhat low gain on a modded TAD JTM45 kit

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vintage_charlie
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Somewhat low gain on a modded TAD JTM45 kit

Post by vintage_charlie » Mon May 10, 2021 2:48 pm

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!

Pictures are below in the links, i could not get the forum to display them as images:
https://ibb.co/cCP66m7
https://ibb.co/F4ZWV17
https://ibb.co/ynLPpqc

danman
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Re: Somewhat low gain on a modded TAD JTM45 kit

Post by danman » Tue May 11, 2021 6:35 pm

Try upping the nfb resistor anywhere from 47k to 100k and see if it gives you a bit more hair. Are you certain the resistors on the ppimv pot are 220k and not the standard 2.2m as used on the LarMar style master volume? There is also a version called the Rich Mod that keeps the 220ks so I'm not sure which version you have there. Switching to EL34's may give you a bit more hair also but that OT has a plate load of 8k where the EL34's want to see something in the 3.4k-4k range.

vintage_charlie
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Re: Somewhat low gain on a modded TAD JTM45 kit

Post by vintage_charlie » Tue May 11, 2021 8:36 pm

danman wrote:
Tue May 11, 2021 6:35 pm

Try upping the nfb resistor anywhere from 47k to 100k and see if it gives you a bit more hair. Are you certain the resistors on the ppimv pot are 220k and not the standard 2.2m as used on the LarMar style master volume? There is also a version called the Rich Mod that keeps the 220ks so I'm not sure which version you have there. Switching to EL34's may give you a bit more hair also but that OT has a plate load of 8k where the EL34's want to see something in the 3.4k-4k range.
Thanks Danman! Yes, the resistors on the pot were 2M2, i misread that earlier.
Will try other NFB values - thats a good hint. I guess 470k mixers might add to gain too?
Meanwhile i reverthed the amp to shared cathode and it was a very big step towards what i would expect from a JTM45 - flubby barely useable gain (when cranked past 8 on the bright channel). The split V1 cathode was actually attenuating and tightening it up (i had assumed that the .68uF cap would add gain, but that was relativated by the 2K7 resistor). So now it is in a similar tone and gain ballpark as my JTM45/100.

To my surprize, i very much liked the tone of the split V1 cathode - might go back to it. It completely changed the nature of the amp.

The OT is actually a JMP-50 style, so i will switch to EL34 soon, as i mod it into a JTM-50.

It has the JTM45 PT though - with about 440v on the plates. Wondering if that is fine for the EL34's.

danman
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Re: Somewhat low gain on a modded TAD JTM45 kit

Post by danman » Thu May 13, 2021 6:39 pm

Glad to hear it's moving in the right direction for you! If you like the split cathode sound, try some different resistor/cap values besides the standard 2.7k/.68uf. See if you like the sound of 1.5k/5uf on each half of the triode.

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