Plexi6v6 voltages wacked

[ToneMerc]

Tonemerc - I had a tube rectifier slot already in my donor chassis, and figured that the lower voltage would act like a little tweak down of a VVR, and wouldn't need to change dropping resistor values from what Mark Huss had. I also saw it as a good thing to get the plate & screen voltages of the 6v6's a little closer to recommended values than what Mark's were. Is my thinking off? If so, what changes from Mark's values do you suggest.
I'm thinking the primary problem here is the low voltages on V3 pins 2, 3, 7 & 8. Anybody got ideas on the next step towards fixing that?
Thanks again.

IMHO, throw the MH voltage chart and power resistor rail values out the window. If you built this circuit pretty much using the same B+ voltages as MH, then it would be a valid guide. However, it's kind of a moot point since you decided to run everything 50V lower right out the gate.


If you followed his schematic, that 20K of resistance between the B & C power nodes is too much. You could use ohms law and run that against your own voltage chart to figure the some of the node values or you could simply shotgun the lower value, try sticking an 8.2K there first. This would raise the entire preamp voltage rail, then to tweak those two 10K 2Ws to taste.

TM

[statorvane]

Lower voltages will certainly result in a browner sound. Can you pop a 5AR4 in place of the 5U4? You'll need to rebias.

You could also try paralleling another 22K resistor between B and C on the 6V6 schematic. That would bring up the voltages for V1, V2 and V3, with minimal change to the power section voltages. You may have to rebias there too.

[deiseldave]

Thanks everyone.

[deiseldave]

[/quote]

IMHO, throw the MH voltage chart and power resistor rail values out the window. If you built this circuit pretty much using the same B+ voltages as MH, then it would be a valid guide. However, it's kind of a moot point since you decided to run everything 50V lower right out the gate.


If you followed his schematic, that 20K of resistance between the B & C power nodes is too much. You could use ohms law and run that against your own voltage chart to figure the some of the node values or you could simply shotgun the lower value, try sticking an 8.2K there first. This would raise the entire preamp voltage rail, then to tweak those two 10K 2Ws to taste.

TM[/quote]
What are the optimum supply voltages at the caps for feeds to PI, and preamp tubes? Thanks.

[martin manning]

FWIW, I have a great sounding 2204-ish amp (EL34) with JMP50 voltage on the plates (~375), and its dropping string is 20k, 10k, 10k, so you may be just fine where you are. The easiest way for you to see the effect of raising the preamp voltages might be to just tack another 20K across the existing one.

[ToneMerc]

IMHO, throw the MH voltage chart and power resistor rail values out the window. If you built this circuit pretty much using the same B+ voltages as MH, then it would be a valid guide. However, it's kind of a moot point since you decided to run everything 50V lower right out the gate.


If you followed his schematic, that 20K of resistance between the B & C power nodes is too much. You could use ohms law and run that against your own voltage chart to figure the some of the node values or you could simply shotgun the lower value, try sticking an 8.2K there first. This would raise the entire preamp voltage rail, then to tweak those two 10K 2Ws to taste.

TM[/quote]
What are the optimum supply voltages at the caps for feeds to PI, and preamp tubes? Thanks.[/quote]

Optimal voltage values at the cap nodes is relative; you chose to wonder off the range from the original design and your voltages are lower. This is no big deal, you just need to adapt accordingly.

You wanted a solution to increase voltage at V3. Thus, if you simply want to raise the voltage of the PI plates and the rest of the preamp plates then decrease he resistance at the power supply nodes. With no other changes to your power supply parameters this is an easy solution to increase the plate voltage in the preamp.

I suggested previously that you decrease that 20K of resistancce to 8.2K, if you have a 10K try that as well. This is your amp, you can tweak those values to get close to the MH values if you wish or to whatever you think sounds great.

Make the resistance change and document the preamp voltages again.

TM

[statorvane]

If you research Marshall 4-holer rail voltages, you'll see the are all over the place. Your's are on the low side, as are Mark's (6V6 Plexi).

The data I have for a JMP50 indicates:

B+1 (Plates) 466
B+2 (Screens) 466
B+3 (V3 - PI) 322
B+4 (V2) 276
B+5 (V1) 250

ToneMerc's suggestion to decrease the 20K dropping resistor between B&C on Mark's schematic is a good one. I'd just parallel a 20K or so resistor to the current resistor, for 10K and give it a shot. It'll let you evaluate the sound with higher preamp voltages pretty quick, for trivial wiring.

[deiseldave]

Thanks for the feedback, guys. Here's an update.
I found a joint that wasn't soldered. The joint that has the PI's (2) 1M & 470R meeting. Soldered it.
Also, going by Statorvane's posted JMP50 cap voltages, I started playing with resistor combinations between B & C feeds. The closest I could get (with what I had) was paralleling (2) 15K's. It gave me (at caps):
C= 327V, D= 281V, and E= 263V. Although a tad high, that looked pretty close to me. I will pick up a 5W 8.2K and a 5W 10K, and one of those should put me right in the zone. I tried the amp out like this, and it is much better sounding.
Also, to hear the effect, I decreased the normal channel bypass to 47mf. I wanted to try an intermediate jump between the 220mf and the 22mf recommended by Statorvane and Tone Lover.
I also upped the 47pf across the PI plates to 100pf per Kdmay's suggestion.
I will have to wait until tomorrow to try out the last two mods. Because, it's kind of late to be cranking the amp.
I'll report back tomorrow.
Thanks again, everyone.

[ToneMerc]

Thanks for the feedback, guys. Here's an update.
I found a joint that wasn't soldered. The joint that has the PI's (2) 1M & 470R meeting. Soldered it.
Also, going by Statorvane's posted JMP50 cap voltages, I started playing with resistor combinations between B & C feeds. The closest I could get (with what I had) was paralleling (2) 15K's. It gave me (at caps):
C= 327V, D= 281V, and E= 263V. Although a tad high, that looked pretty close to me. I will pick up a 5W 8.2K and a 5W 10K, and one of those should put me right in the zone. I tried the amp out like this, and it is much better sounding.
Also, to hear the effect, I decreased the normal channel bypass to 47mf. I wanted to try an intermediate jump between the 220mf and the 22mf recommended by Statorvane and Tone Lover.
I also upped the 47pf across the PI plates to 100pf per Kdmay's suggestion.
I will have to wait until tomorrow to try out the last two mods. Because, it's kind of late to be cranking the amp.
I'll report back tomorrow.
Thanks again, everyone.

At that position a 5W is overkill, a 2W 8.2K would be fine.

TM

[Roe]

60s jmp50s with the standard 118 PT (not the exceptional 133) usually had voltages around 415-20v, given correct wall voltage and bias. In the 70s marshall lowered the voltage to around 380v.

[deiseldave]

Got a chance to play the amp, today. Sounds really good. Way better than before. I can notice the change in the normal channel since reducing the 220mf to 47mf. I don't think I want to reduce it anymore. I like having a little bottom on that channel. I didn't notice the PI plate change from 47pf to 100pf as much. Perhaps if I switched back and forth, I would.
So it is now sounding about like I would expect the amp to sound. However, at high volumes it is producing the type of feedback that microphones do, rather than morphing into harmonics like I want it to. Any ideas on that?
Thanks.

[statorvane]

May mot be a problem with the amp. Are your pickups potted? Mine will generate feedback pretty easily, but any squeal resulted from unpotted pickups. Once I potted then (cheapo Aglie PUs) they easily hang in there.

[deiseldave]

May mot be a problem with the amp. Are your pickups potted? Mine will generate feedback pretty easily, but any squeal resulted from unpotted pickups. Once I potted then (cheapo Aglie PUs) they easily hang in there.

They are potted. It happens even on my EMG's. I'm going to swap tubes around at lunch, and see if it gets better. Thanks.