Marshall 50 watt JCM800 PT\Heyboer Woes

[tork]

I recently had a power transformer go out on my 1985 50 watt JCM800 2204 head. I was about to order a replacement for Antique Electronic Supply when I found a few threads with people claiming to have been having problems with their part while singing praise for the Heyboer. I ordered a Heyboer from Mojo (for $22.50 more than AES wanted for theirs) hoping to get this fantastic sounding amp back in working order without any issues. The new transformer showed up and I installed it along with a fresh new set of filter caps. When I powered the amp up I immediately noticed that the B+ was considerably lower than I expected with just the bias current being drawn and I was barely able to get 40 watts output @ clipping. I checked another 50watt JCM800 I have and was easily able to get 62 watts @ clipping on the same test setup. The plate voltage on the Heyboer tranny amp dropped well below 400 volts under load. I used a Variac with ammeter to power the amps and the unit in question does not draw the current it should (about half what the *good* amp draws @ clipping). I can find nothing wrong with the amp except for a weak power transformer. I spoke with Mojo about this and while they claimed it would be highly unusual to get a bad transformer, they could not think of any other reason my voltage was sagging so much that I could not get the power out the the amplifier I should be. They asked that I jump the output transformer from my good amp to the one in question to make sure it was not bad as well - same exact results. I shipped the transformer back to them and it was tested quickly with a 10K resistive load on the high-volage winding and deemed *good*. This simple test is far from conclusive and I cannot seem to find anything in the amplifier that could be causing the problems. I did try playing through it with the Heyboer replacement and immediately noticed the lack of volume, punch and headroom it had before the original transformer blew up.

I'm still waiting for Mojo to look into this further, but I am currently out the cost of the transformer and shipping both ways.....not to mention the fact the I'm without the use of one of my best sounding amps for almost a month now! Any help/suggestions would be greatly appreciated!

Just to clarify a few things...

I have an EE degree and have been working as an engineer or technician for the last 19 years. I normally perform all the work on my five Marshall amps without any issues whatsoever. I triple checked my wiring and verified that the 120V tap was being used and there was no voltage drop to the primary (filament voltage seemed fine as well). The rectifier diodes all appear OK and the voltage sag showed up on the AC side before the diodes. The standby switch is good and there was not any excessive drop across the filter choke - it just seems the high-voltage winding does not have enough ass. The amp will not draw much current on the Variac ammeter no matter how hard I drive it. I have tried several sets of new 6550's all with the same results. Measured RMS voltage @ clipping across a non-inductive load with a Fluke 8060A True RMS meter etc. etc. All this and the Heyboer mounting holes didn't line up worth a damn as well!

I'm open for any suggestions as to what could possibly be wrong with this amplifier - I still maintain that the transformer does not meet spec and I'm trying to find one locally to try out to prove it.

Jack

[Amp Kat]

Have you measured the current draw of the transformer to verify it's validity. The Marshall tranny may be a higher current tranny than the Heyboer. If it is for instant a 300-0-300 tranny @100 ma's ? then see if it's drawing that current. What is the current draw of the tubes and does it bias up right at idle with no problems. You replaced the caps so is it possible you have a bad cap maybe ? I've had out of box failures on those also.

[Rick Erickson]

This is my third attempt at a reply...
You have a PT modeled after the voltages of an early 50W Marshall. (The "desirable" ones everybody wants) When Marshall added the 1K screen resistors to these amps the power dropped to ~36W RMS due to reduced screen voltage at full output. There are a LOT of 50W Marshall amps that only produce this amount of power. You can bump it up a bit closer to 50W by reducing your screen resistors. I would not go lower than 200 Ohm, 5W - unless you like to watch your screens melt. Or you can get a transformer designed for a modern 50W - Or you can live with it as-is...

RE

[tork]

Well....lowering the screen resistor value is something I would rather not do. This lower voltage transformer may sound great and be *more desirable* in certain model Marshall's, but it surely does not sound nearly as good in this 1985 JCM800 as the original did.

It is starting to sound like I may have to go to Mercury to get something that will sound like the original, which is a shame as their prices are quite steep!

[hasserl]

I suppose "good" is objective. It doesn't sound like you want it, so that's what matters. But Rick is right, the JCM800's I've seen also had the lower secondary voltages, i.e. less than 400.

Looking at AES's tranny for that application looks like it should get you the voltages you're looking for. I looked at the trannies on Mojo's site and they don't give the spec's. Looks like you'd have been better off getting the AES tranny.

Before buying a Mercury tranny you'd better check what the secondary voltages are or you could end up with the same situation you have with the Heyboer.

[dai h.]

yes, make sure because Marshall 50W PTs came in a variety of voltages which one can gather if you look at their 50W PT range, which varies depending on the particular unit. I have two different (385V '80 JMP MasterVolume and 450V '82 JCM MV) plus I've heard on some older ones as low as 360V. The recent reiss. Marshall seems to be the same (about 450V depending on wall voltage)--I have one of these--plus I can tell you that the New Sensor one is also right about the same (one of these also). So confirm to make sure you get what you want. I suppose you could try to ebay the low B+ one or something.

[Rick Erickson]

You can lead a horse..

Well....lowering the screen resistor value is something I would rather not do.

Then I guess you will need to spend the dough for another transformer... I would at least try it and see if it works - what's it gonna cost you, $2.00 - $5.00 in parts?
I do hope you realize that my use of the quotation marks around the word "desirable" was to point out my cynical use of the word. This is yet another example of people cloning things without fully realizing the limitations of the piece they are cloning. I have used this (reduced value) screen resistor trick in several 50W Marshalls, including two JMP 50W MV 2-12 combo amps that I have owned & modified. But if you don't want to try it...

[tork]

I didn't want to lower the value of the screen resistor only because it may not be as safe (?) if I have a tube short out on me etc. Also, I've already sent the transformer back to Mojo anyways (I was thinking it was bad - but it now just appears that they choose a lower voltage secondary than I was expecting) so there is nothing for me to try at the moment.

I spoke with Mercury and gave them the part number from my original trannny to get the exact replacement model (P50JM-475). They make a number of Marshall 50 power transformers to match the voltage of the particular model and vintage (so the amp will sound the same as it did with the original). Seems like a great idea aside from their high prices.

Thanks for all the help guys - I doubt there is much more anyone can help me with at this point. I just couldn't figure out why the Heyboer was not performing like the original tranny and assumed it may have been defective. The amp did work with it, but sure didn't sound anything like it used to and was certainly not what I was expecting.