JCM 800 2204 50W High Plate Voltage question!

[jmp50_1967]

Hi.

I am new to this forum so please bear with me.  I would appreciate any kind of help! 

I built a jcm800 2204 clone with classictone (Magnetic Components) PT and OT.

On my first fire up I tested the plate voltage and noticed it was a lot higher than what other people were saying on forums.  It was 507V!!
I've seen closer to 450 to 480V which seems to be the right range.
I measured this with one probe to the chassis and one to pin 3 of the power tube.

I am sure I wired everything up correctly.  It is very loud in fact. 
I was told that I maybe had wired it wrong and that something wasn't using the voltage hence the high plate voltage.  But I am sure I didn't.  Initially though I did have it unorganized but I decided to restart the whole thing and give it another try.  It is basically the same issue.  High plate voltage.  I don't think 507V is normal.

I have heard though that classictone in fact winds their trannys higher for some reason. 

Any thoughts on this would be greatly appreciated!

Thank you.

[shooter]

I'm guessing you're USA Volts, 120?  might check to see if that's high.  Here's a link to the schematic on dougs site, with voltage readings, does the schematic match yours?  If so, what are some of the other readings compared to this schematic?

http://el34world.com/charts/Schematics/files/marshall/Marshall_JCM800_2204_Lead_Series.pdf

[John]

I could be totally wrong (probably am) so take this with a grain of salt: but if it sounds good, and there's no weird noises, and the tubes aren't redplating, I wouldn't worry too much. You're about 10% high on the plate voltage, and if CT winds theirs a little hot, there ya go. The main thing to check is that you're not biased too hot. Too much current  kills tubes faster than higher voltage, within reason of course.


It's Easter, so it might take a bit longer, but someone who knows a lot more will soon be along and give you good advice.

[jmp50_1967]

Thanks John and Shooter!

Yes. I am in Canada so the wall voltage should be around 120V.   
I have checked that same schematic before on all the preamp pins and power pins and they seemed to be within range but not exact. But I will have to check again..  By any chance would there be a difference in gain or tone when comparing high to low plate voltage?  I have heard things where lowering the plate voltage could create more saturation or compression.  It seems like mine has a lot of headroom. Even with the preamp all the way up it doesn't seem to produce too much gain. I have tried a stock jcm800 in the shop and it seems to have more gain.  so not sure if the plate voltage has anything to do with it.

I was going to take this one step at a time and start with a simple plate voltage question, but I have encountered several very strange things with this amp previously before i redid everything..

1)  I tried changing out the preamp volume pot with a higher quality PEC pot.  There was a significant reduction in gain.  I made sure they were both the same at 1M log.  I even swapped out different PEC 1M pots and they all did the same.  Only difference is that they are rated at 2W vs the 1/4W Alphas.  Still does this even after rewiring..Weird....

2)  One time I tried swapping the 470K resistors on second preamp stage with a a lower value.  swapped them back to 470k and upon firing on the amp, capacitors burned open with a sound of a sizzle.  Not good and weird.

3)  Upon changing out the preamp pot one time there was smoke that came out near where the power tubes are.  No sound.  No shorts in the power tubes.  Problem fixed but never found out what happened or where the smoke came from.

4)  Wanted to add more gain to the preamp so I bypassed V1a 10k resistor with 0.68uf cap.  Sounded great for awhile until I swapped out a preamp pot from Alpha to PEC to CTS.  All same values.  Then upon changing to CTS there was a fuzz type sound with the preamp all the way up.  Literally sounds like a Fuzzface.  When hitting lower strings there is this sort of overload effect or clamping where it seems like a capacitor cannot discharge fast enough.  Changed back to the original Alpha pot but the fuzz was still there and never went away.  It was definitely not there before is all I can say.  Really weird. 


Most or all of these problems occurred where I swapped out the preamp gain pot.  Possible DC leakage to pots??  I swapped out all the filter caps btw, but that didn't seem to help. It was just plain strange.  Have any of you had an experience with this?  I feel at this point that JCM800s and me are just not meant to be lol.

I already replaced everything on the board.  plus all pots and sockets.  And even the power filter caps. Rewired the whole board with new wire.  The ONLY things that I haven't changed are the transformers and choke. I did this to make things neater.  To be honest it was a mess so i guess it was worth cleaning things up.

So that fuzz problem is still there and the only things I can think of are the transformers.  Which lead me to looking at the PT plate voltage.  Because right from the beginning I felt this was high and possibly the source of all my woes previously mentioned. 

Thanks again for your kind help..





Thanks again!

[sluckey]

It was 507V!!

That reading is with all tubes plugged in, right?

How do you have the bias set? Biasing tubes to run cooler will cause the plate voltage to read higher. Biasing hotter will cause the plate voltage to decrease.

[jmp50_1967]

Thanks Sluckey.

I tested the plates upon first firing up the amp.  First I checked the plates on pin 3 of the power tubes to chassis with all tubes plugged in.  This was 507V.

I did the calculations as stated for EL34s and found the nominal value to be around 34mA with the plate voltage that I had.  So from there I biased power tubes to draw about 34mA.  I haven't really checked plate voltage after biasing it hot however.  But it seems like it has always stayed at 507V whenever i tested it.  Even with the bias hot or cold.  Maybe I'll have to really check to see if it even changes when biased hotter.  But if I want to lower the plate voltage I would have to bias a lot hotter which would mean the tubes would not be at nominal current draw?

What is the correct step by step method of measuring plate voltage then.  Because I was always under the assumption that the plate voltage always stayed the same whether biased hot or cold.  And that the nominal current draw is calculated based on that plate voltage.  Or I must be missing something here.

Sorry for these noob questions..

[sluckey]

Anytime you adjust the bias you need to recheck the plate voltage.

[EL34]

Here's my follow up to the email you sent me


What is the actual wall voltage?
That changes everything

What is the bias current per power tube set at?
That changes everything

[jmp50_1967]

Thanks for the follow up!

I just checked the wall voltage with my digital multimeter.
For some odd reason it's showing in decimals..wweird.
But on the right side of the plug and the ground it shows very close to 120V (0.120V) to even 138V (0.138V)on the left of the plug shows about 90 to 100V.

This seems about right?

[shooter]

This seems about right?

If you're getting 138 that's gonna raise B+

What is prong to prong, measure Vac across the prongs

[PRR]

> 120V (0.120V) to even 138V

I do not know whatsup with the decimal.

Measure between the two flat-prong holes (not to the U-ground hole).

At my house I run 125V to 108V depending on load. (I live far back from the street).

Anything much over 125V makes trouble. At 138V your incandescent lamps will run bright and burn-out very fast (6 times faster than rated). While I have heard of houses running 130V and rising, I heard of them because there was trouble, and the power company had to fix their problem. In one case the 120V was pretty close all day but rose after midnight when everybody turned off the lights and TVs

I do not know that 507V is "wrong". It's just 10% higher than you expect for a 320V winding running near full load. You DO need to check again after roughing-in the bias. You do NOT turn-up idle current to suck-down the B+, just aim for 70% Pdiss. 35mA should be fine.

[drew]

I just checked the wall voltage with my digital multimeter.
For some odd reason it's showing in decimals..wweird.

Maybe it's trying to tell you it's new battery time?

[jmp50_1967]

Thanks everyone for you help.
yeah it was weird that it was in decimals.  I think the two probes I was using were not long enough and probably not touching the two prong holes properly to get a good reading.
I decided to check inside another 5f1 clone.  My 2204 has been in the shop for about a couple of weeks to check the trannys.  If he's too backed up I'll do him and me a favor and take it back.  This forum has been very helpful. 

I probed the green and black wire of the power cord and it read 121.5V.  I saw it drifted to about 121.7V to 121.4V.  But it seems to be steady for a bit at 121.5V.