Help me understand V3 on JCM 800 plate volts

[chabby-ao]

In many schematic of JCM800 2204's I see a charted voltage of around 390.0VDC at the junction of the pin 1 and 6 of V3 82K/100K junction, but then at the plates is a way lower voltage. I understand that the voltages will be lowered by the respective resistors, but I'm seeing cartd volts 150 VDC lower at the plates on V3.

I'm not getting that on my JCM 800 build, I'm getting higher voltage at the plates than at the 82K/100K junction, or close to the same. my voltage is about 398 at pin 6 and 400 at pin 1 which seems way too high. The amp sounds great, but where an I going wrong? I know the resistors are to spec.....how does one person get 150 VDC lower with the same voltage going to the balancing, or rather out of balance part of V3?

[sluckey]

The voltage on the plates should be considerably lower than the supply B+ voltage at the junction of the plate resistors. It's impossible to have more voltage on the plates than supply B+ voltage.

Since your plate voltages are high, it seems the tubes may not be conducting enough. Tell us what voltages you measure on pins 2, 3, 7, and 8. Also, be sure pins 3 and 8 are tied together. Then, with the amp turned off, measure resistance from pin 3/8 to ground. Put one meter lead directly on the tube pins and the other on the chassis where the PT center tap is connected. What are your numbers?

[chabby-ao]

pin 1=406

2=288.6
3=11.20
6=406
7=287.8
8=11.20

[sluckey]

Are you sure about pins 2 and 7? With only 11 volts on the cathodes, that tube should be melted!

What about the resistance measurement? It's important.

[chabby-ao]

Okay - I got 497K resistance

and I'll remeasure 2 and 7 to be sure...................
Okay 291.6 +-10% on 2 & 7
11.18 on 8&3

Just for kicks I measured my 470K resistor going to the cathodes and only measured 43.1K.....could that be the problem? Or, can I trust that measurement in circuit?

[sluckey]

Okay - I got 497K resistance

That's the problem. Take that 470K resistor off the cathode and put a 470 ohm resistor in it's place.

[chabby-ao]

Think I just solved the problem as I had a 470K R in that slot instead of 470r!!!!

Pays to triple check....dumb me....

Okay changed it and it dropped everything, but now I have some other issues I need to solve. I'll post some new voltages in a minute.

[simonallaway]

When I get home I'll post voltages from my own 2204 as well as some from a book I have. Those might help.

[plexi50]

Okay - I got 497K resistance

That's the problem. Take that 470K resistor off the cathode and put a 470 ohm resistor in it's place.

That 470K sneaks in there and does it everytime  

[chabby-ao]

voltage now at pin 1 255.7 and pin 6 = 248.4

8 & 3 are 38.0

However now my problem is best way to describe is like it's too touch sensitive and it's making the speakers react poppy. Is that most likely a tube issue?
Help me out on this hopefully last part of the witch hunt. Everything is looking better and I've got loads of volume, just some poppy issues to isolate.

Plates of power tubes are running at 467 with bias voltage at -54.8 haven't installed any 1 ohm r's yet to calculate diss, but will shortly. I got a bad bunch of svetlana EL34's with only 50% not bad. Once I culled through those to get a couple good ones, I think the PT's are okay. We'll see.

This is pretty unique because I started from a JCM800 2210 and converted it to 2204. It has it's own bias tap which is different than 2204's and I had to configure it for a full wave bridge rect. Could the poppyness be due to using a 100K negative feedback r ? I've got it running to the 4 ohm tap. Any ideas?

The 2210 used a 100K r that joined with a 1.5K that split off but the 100K was in series with a 5K - I really have no understanding of the 2210 and this 2204 is much simpler and is on the verge of sounding killer once I solve this last issue(s).

What is the formula for figuring negative feedback resistance? This is the first Marshall of this size and type I've messed with and has the potential to be killer, but I need a little help yet.

[chabby-ao]

Okay - I got 497K resistance

That's the problem. Take that 470K resistor off the cathode and put a 470 ohm resistor in it's place.

That 470K sneaks in there and does it everytime  

Yep - I should really know better than that. It solved many problems and let me get on to solving some better problems - help pleeze!

[plexi50]

You got it right. The plate voltages and PI #3 & #8 look good. Now to bias it and you should be done. A bias that is too cold can sound muddy and distorted. I like to bias my amps some where in between hot and cold. Course that also depends on the amp your using. 2204 bias it a little on the hot side

[sluckey]

The 2204 is a high gain amp. Lead dress and layout are critical. The pics you posted on another thread indicate that lead dress and layout may be contributing factors. There's a LOT of room for improvement in this pic...

[chabby-ao]

I totally know that - and knew I'd take the heat for it, but oh well.
All I had were scraps for this project, but couldn't wait. Combine that with being in a hurry because I have to go away for awhile and there you have it. That said, it is doing pretty damn good for that lay out and lead dress. All I had was a piece of wood, some terminal strips, a couple screws and an old to small organ board and I knitted it all together. That it works at all is a miracle and that it will end up sounding killer once I bias it up is even better, Just trounces the tone of that 2210 in spades. Ballsy deep bass and AC/DC like steroidal butt kicking.

The only reason I posted that picture was to encourage someone fighting with a 2210. If I can do it with shit laying on my floor, anyone can do it. Every part you are seeing in that picture was picked up off the floor, or ripped from some other amp. I even had to piece together resisitors at times. The bus bars were old and used multiple times. There is nothing new - have you ever ripped stuff off amps? It ain't easy, then cleaning stuff up is even harder. I've put together some pretty amazing amps from crap like this. This pic doesn't even represent a finished amp anyway as I know about lead dress and Marshalls.

Some things they do don't even make sense on their boards. I much preferr to go entirely PTP, but had no money and no stuff to go that way completly - I hate terminal boards. Boards dictate everything from layout to ground and lead dress, whereas PTP things make more sense, grounds are more intuitive instead of going where they have to go. Circuit design on these Marshall high gain amps is borderline ridiculous with things criss crossing that never should be done.

But oh well, this ain't no show piece that's for sure, but if it works like I know it soon will, I'll add it to another list of scrappy builds that are fugly, but sound killer-lol! I will clean up the lead dress however now that I know for sure everything works as is. Can only get better from here - I'll shorten up some leads too. The amp is very quiet and has been even when I had that wrong resistor feeding the cathodes of V3. I'm actually shocked how quiet it is for such a high gain circuit, so I must've done something right. I think my ground scheme is very good. I don't do star completely. I like to ground all the power components on the power side as typical, but I like to ground from mid board to inputs either right where they are and anchored near the inputs - always makes things quieter it seems.

[sluckey]

Please don't take my comments as a personal attack. It's just an observation. High gain amps need special attention.

Good luck. This amp is a couple steps up the ladder from an AO-43 conversion.

[chabby-ao]

Funny you should mention AO43 conversions. I had one that went from a Marshal style 18 watt, to a 5E3 to kind of a hybrid swictahble unit that can use both 6V6's and sound like a black face Princeton, to 6L6's and sound like a tweed pro or extra strength 5E3.

That amp was a wonder and I ended up selling it to a local pro who runs it dual along side his black face deluxe - he loves the amp as did I. It didn't look so pretty either, but damn it just dripped with mojo. I love the iron in those old AO43's.

I finally ended up junking the AO43 boards and going totally PTP in that small chassis. Then even that tiny chassis seemed large-lol! PTP doesn't require the cavernous chassis terminal boards do.

[sluckey]

Funny you should mention AO43 conversions.

It wasn't a coincidence. I remember the threads well.

...sluckey, aka 57chevy

[chabby-ao]

I 'm not 100% sure or not....were you the other guy that did one?
I know there was a guy that did the exact same one I did but I think his name was schottky or something like that. His was even messier than mine, but both worked well in the end. I loved that AO43 because it was allot of fun to experiment there were several ways to go with it. I left it just open for almost a year or so, tweaking it constantly. I will admit it found it's calling more along fender lines than Marshall and ended up doing better with either the standard 5u4GB it came with or 5Y3. When I sold it the guy liked it with both rectifiers so well he ended up buying all 3 GZ34/5U4GB and 5Y3.