JTM45 rectifier question

[Roe]

the flying resistor gives more screen sag and a somewhat softer attack and slightly reduced output. I like at least 470R as a flying resistor on a jtm45 with 445v on the plates.

[Reeltarded]

What do we think is insanity for that resistor value? I have a similar power section and a case of resistors I have been staring at. The only think this amp lacks is the squat that makes the amp way more vocal. I was thinking of raising screens to 2k2.

Sorry for temprary derail.

[martin manning]

What do we think is insanity for that resistor value? I have a similar power section and a case of resistors I have been staring at. The only think this amp lacks is the squat that makes the amp way more vocal. I was thinking of raising screens to 2k2.

I think you'll have to experiment a bit to find out. Maybe put a 2k2 or 3k3 in for the flyer, and parallel others across it until you find something you like. IMO the primary impedance on JTM45's is way too high, and this extra screen dropper/limiter is compensation for that.

[Reeltarded]

I think you are right about the primary being way too high. A little high is pretty good though.

I am going to hit it with a bat and back out of it so I don't miss something special. I get funny when everything is almost perfect.

[SPeter]

I got carried away last night as I got a new speaker which finally let me dime everything LOL. I got an Eminence Reignmaker which has the FDM function so I could dial things back a bit. Sounded pretty good even turned down to 91dB efficiency. I was having way too much fun to get back to voltage checking...

... I’m interested in your voltages when you get them. You are running 8k primary impedance, correct?

.

I have the classictone 40-18039 output transformer and its documentation ( http://www.classictone.net/40-18039.pdf ) shows 8K Ohm on the primary side so I think the answer is yes to the 8k Ohm Primary. 1K's might be in Tonight then it is voltage checking time.

I noticed something concerning last night though. The whole chassis is microphonic. I tap it and I can hear it in the speakers. I can also hear the pots making a microphonic noise when turned.
I'm not sure this is related but with all loudness turned to zero I still can hear the guitar sound coming though the speakers very quietly. I'm not sure if it was like that before I installed the larmar :O. The larmar dialed to zero will stop this behavior.

[SPeter]

I put in the 1K resistor tonight and measured up he pins. I'm not sure I can tell the difference with the flying resistor on. I love how this thing breaks up. It destroys anything I can dial in on my Egnater Tweaker 15. The amp sounds real nice but when everything is dimed it gets a feedback when I'm around 2 feet from it with the guitar on me... I'm not sure that this is normal. I worked the cables of the LarMar and the microphonics got a little better but it is still there. It is most pronounced when tapping the input jacks and the chassis around the V1.

Anyways, the pin voltages:

pin voltages by cvt01, on Flickr

[Reeltarded]

Still pingy when you tap the chassis? Swap PI tube into first position.. or better yet, tap lightly each 12ax until you find the source of the ping. If its V1 (and it is, just swap that tube with V2 and see.

[martin manning]

Thanks for posting your voltages. I'm surprised that the screen voltage (pin 4) is the same with and without the flying resistor. I would expect a drop of ~10V relative to the anode (pin 3) with that resistor installed.

[Roe]

the new chinese kt66s dont draw much screen current but RFT/Siemens el34s do

[martin manning]

the new chinese kt66s dont draw much screen current but RFT/Siemens el34s do

Can't be zero, though!

SPeter, the big change in pin 5 voltage looks odd too.

[SPeter]

I don't know what to tell you Martin. The voltages start to drop towards the circuit board after every resistor if that made any sense. The byoc layout is different from the metro so that the 16uf cap has a jumper to v5's pin 6, i wonder if that has to do something with it. I'll recheck everything tomorrow.

I switched the mullard from v3 and the sovtek lps in v1 and the microphonics is gone. So it seems I got a bad tube from TubeDepot. I have a second sovtek in my other amp I will test that one in v1 tomorrow just for fun.

I got to play around with the larmar combined with the Eminence speaker and I'm impressed. I can dial back the volume without ruining the sound of the amp a lot. It is not bedroom levels but really great basement levels... lol.

[Reeltarded]

the new chinese kt66s dont draw much screen current but RFT/Siemens el34s do

Can't be zero, though!

l o l oooOOooooOOooOooo.... it's spooky. It's far away. It's spooky, it's action... it's at a distance..

[martin manning]

I'm not superstitious... :^) I think it's the result of blindly mixing layouts. Just connecting the 1k where BYOC shows it has it shorted out.

Metro shows a 1k from the screen node to the screens in the nicked Marshall schematic, but the wiring instructions don't include it. The Metro instructions result in what I'm showing below, with the 1k grayed out. To work in the 1k some rewiring will be needed, placing the 1k between the screen filter and V5 pin 6, and connecting the 8k2 tube end directly to the screen filter.

SPeter, you don't need to go crazy tabulating voltages, but how are you setting bias? You should measure and record the power tube pin 8 voltages so you can see how much current is flowing through each tube. Then big change in pin 5 voltage might be explained.

[Reeltarded]

I have literally NEVER seen that shared resistor in anyone's 45 and only know that Roe did it because he said he did.

[SPeter]

I'm not superstitious... :^) I think it's the result of blindly mixing layouts. Just connecting the 1k where BYOC shows it has it shorted out.

Metro shows a 1k from the screen node to the screens in the nicked Marshall schematic, but the wiring instructions don't include it. The Metro instructions result in what I'm showing below, with the 1k grayed out. To work in the 1k some rewiring will be needed, placing the 1k between the screen filter and V5 pin 6, and connecting the 8k2 tube end directly to the screen filter.

SPeter, you don't need to go crazy tabulating voltages, but how are you setting bias? You should measure and record the power tube pin 8 voltages so you can see how much current is flowing through each tube. Then big change in pin 5 voltage might be explained.

I followed the instructions when setting bias. With my plate voltage I got 38-39, I set it to 39.

I think you re correct. I followed the Metro instructions. The yellow wires go from 32uf cap to V5 then to the V4 then they used to go direct to the circuit board. Now there es a 1k resistor there instead of the wire. I don't think it does anything...
Capture by cvt01, on Flickr