Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on May 31, 2021, 08:01:53 am
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Ok here goes...
took the schematic from Matec and added a TMB tone stack. Also want to add a MV, so I drew 3 scenarios for that. The amp has a cathodyne PI so there are some options.
Option 1: is the MV right behind the tone stack. The disadvantage is that it's before the last stage of amplification. I feel it's a better option after that stage.
Option 2: is the MV right before the PI. I think I have to eliminate the NFB on the schematic to do that? I read it's the better option for a cathodyne PI for it will give me control over the infamous cathodyne double frequency blocking distortion? Is there a way to maintain the NFB loop in this configuration?
Option 3: is the post PI MV, trainwreck type 3. I can maintain the NFB loop if it's in that spot. But then again, will I call for the blocking distortion after all?
Isn't that NFB loop cleaning things up too much. Note that it IS on the schematic, so.. it must have it's advantages?
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1. I'd connect the treble wiper to the top of the MV pot.
2. MV will not work. It interferes with the PI bias.
3. Crossline MV works but I don't like it. Lot's of others agree. There are better MVs.
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1. I'd connect the treble wiper to the top of the MV pot.
2. MV will not work. It interferes with the PI bias.
3. Crossline MV works but I don't like it. Lot's of others agree. There are better MVs.
I'm biased towards option 1.. but is it effective enough? I wonder if I'm not better off with that TMB tone stack as a cathode follower, since I replaced a single tone pot with that TMB... signal loss wise...and then that MV after that
Is the lar mar one of the better MVs you're referring to?
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If you use a CF to drive the TS you will probably want to add an extra gain stage. LarMar is much better than crossline IMO. I also like the MV in option 1 with the correction I noted above.
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I would love to see a board layout version of this amp ... Will definitely be following this build. I would think a Lar/Mar MV would work great in this circuit.
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ok, so I removed the MV aftre the tone stack and added the Lar Mar MV. I think this should be a correct schematic so far...
and the matching layout.
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Looks good, don't know if it makes any difference, but with what looks like 100K grid resistors on the phase inverter, should you use a dual 100K pot for the PPIMV? Just throwing that out there.
Also, any need for a presence pot to control NFB?
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The presensce control ain't on the original schematic either, so I'm not gonna bother.
The 100k dual pot... True enough Yes.
Good point. I got too tied up in the lar mar schematics I must have overlooked.
Anyway, some of the parts arrived today.. did the preps for the turretboard.
But I'm doing lots of stuff lately. Just finished my new telecaster today too.
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May be it isn't so important but ....... I'll move that MV pot on the left side of the tone controls
near the bass pot, as to have shorter wires
(https://el34world.com/Forum/index.php?action=dlattach;topic=27582.0;attach=92077;image)
Franco
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May be it isn't so important but ....... I'll move that MV pot on the left side of the tone controls
near the bass pot, as to have shorter wires
(https://el34world.com/Forum/index.php?action=dlattach;topic=27582.0;attach=92077;image)
Franco
I know,but I'm using a standard faceplate from the TMB lite ( modulus amplification).. so that's not an option
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Need Help here... there is a sub octave note on the amp. Where do I look?
When I increase gain it's less audible. ( not on the recording)
I tried adding a parallel filter cap to get 55uf on the 1st filter, but no help in that..
It is on several different speakers, so that's not the problem.
Also tried different preamp tubes, but kot the 12AU7 power tube.. I don't have a spare
https://www.dropbox.com/s/i2jmhgc0wi1dov1/Dissonant%20tone%20jtm145.m4a?dl=0
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The heater circuit doesn’t look to be well decoupled from the audio path. How about trying a bigger bypass cap, or referencing it to 0V common?
I don’t see the point of a master volume after a cathodyne? As it’s got no gain and can distort yucky. I use option 1, but wire the master as a pot.
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It's not that I guess... I never referenced the heater CT to the cathode, but to star ground.
I tried reducing the low end in various places because it was pretty muddy at first.
Is it possible the cathodyne PI is distorting and the sub octave is the result?
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Not the filter caps.. parrallelled all 3 x 33uf with 22uf ...should be plenty
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If it’s muddy, why retain the filter across the OT primary?
Is the nfb actually doing anything (at high master vol settings)?
Are you sure it’s not positive?
What was the master vol setting for the soundclip? Does the master setting alter the character of the distortion?
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I mostly followed matec's schematic, including the filter across the plates of the 12AU7. Maybe I should try it without..
Removing the NFB loop increases OD a little, but doesn't change overall tone, certainly not the ghost notes.
I even swapped both OT primary wires just to check... nada
Don't remember the vol setting, but the subnotes are always present, no matter what setting. Only with gain and vol dimed it's a little less obvious.. probably kinda buried in the overdriven sound.
When the MV is rolled back the sound gets thinner, but still sounding like a ring modulator.
Yesterday I injected the signal from the preamp into the power amp of an 18 watt and vice versa. Though the preamps don't match I have the impression the problem is in the preamp of the jtm145.
Now might be a good time to bring on the Siglent 1102 CML
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Problem solved.
Turns out I have been very very stupid. The amp has an attenuation switch that selects between 1W and 0.1W .. it was in 0.1W mode all the time.. and we all know that this kind of attenuating is tonally not the best option. Yesterday I was checking out how quiet it would be at 0.1 watt, but when I switched it was a LOT louder...and the sound was superb. I have been tweaking and soldering all week, finetuning... and when I was finally satisfied more or less I found out the switch was on 10% mode...lol
https://www.dropbox.com/s/jqt0i2dgi7vtq2e/jtm145%20omg%21%21%21%20this%20is%20awesome%21%21%21.m4a?dl=0
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The NFB loop on this schematic just seems odd to me, the ones I'm familiar with go into the cathode of the driver circuit...
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The NFB loop on this schematic just seems odd to me, the ones I'm familiar with go into the cathode of the driver circuit...
I know... but that's how it was probably built in the first place by marshall. I think Matec's analysis of the circuit was pretty profound. Maybe it's something to do with the PI not pushing the 12AU7 tubes too hard?