1982 Marshall 4210 PT Voltage

[plexi50]

I have a 1982 Marshall 4210. My red leads feeding the rectifier diode's show 300VAC before rectification. Each red lead has 300VAC on them.
After rectification i have 400VDC. This is wrong IMO. I should have 400VAC & B+ of 500VDC unloaded. 470VDC + loaded with a pair of 6550's.
I don't want to believe the Drake 1202-324-2 PT is going bad. Can you think some thing i am missing here?
Standby switch is good and i have also removed the red leads from it to test the PT directly.

385VDC with 6550' wont get it.

[Ed_Chambley]

You know much more than I, but I do have amps with these transformers.  Mine all have 120v (Black) 220v (Orange) and 240v (Yellow).  Are your primaries matched to your wall voltages?  Just guessing.

[John]

It it's not that, could it be a bad CT ground?

[plexi50]

Yes it has 120VAC - 220 - 240. The primaries are matched to the wall voltage of 120VAC.  Wouldn't the voltage double to 600VDC if the CT were ungrounded? Is the CT the black wire a i see on the schematic?

[John]

Yes it has 120VAC - 220 - 240. The primaries are matched to the wall voltage of 120VAC.  Wouldn't the voltage double to 600VDC if the CT were ungrounded? Is the CT the black wire a i see on the schematic?

I was thinking backwards anyway. If the ground was dodgy, that would raise the "0" from ground potential, not lower (like you do with a zener to drop B+) Ignore me! 

[sluckey]

I looked at the 4210 schematic found on this website, and found the amp series had a bridge rectifier.

You looked at it wrong. This amp only has 2 output tubes and uses a conventional 2 diode full wave rectifier, not a bridge.

I have a 1982 Marshall 4210. My red leads feeding the rectifier diode's show 300VAC before rectification. Each red lead has 300VAC on them.
After rectification i have 400VDC. This is wrong IMO. I should have 400VAC & B+ of 500VDC unloaded. 470VDC + loaded with a pair of 6550's.
I don't want to believe the Drake 1202-324-2 PT is going bad. Can you think some thing i am missing here?
Standby switch is good and i have also removed the red leads from it to test the PT directly.

385VDC with 6550' wont get it.

How does the amp sound? 300VAC may be a bit low for a 50W amp, but 400VDC after the recto is right for 300VAC. Does the PT get hot? Put a pair of EL34s in it. Did the B+ increase? Is there anything wrong with the amp other than you feel the B+ should be higher? Maybe Marshall built a conservative amp for a change...

[plexi50]

I looked at the 4210 schematic found on this website, and found the amp series had a bridge rectifier.

You looked at it wrong. This amp only has 2 output tubes and uses a conventional 2 diode full wave rectifier, not a bridge.

I have a 1982 Marshall 4210. My red leads feeding the rectifier diode's show 300VAC before rectification. Each red lead has 300VAC on them.
After rectification i have 400VDC. This is wrong IMO. I should have 400VAC & B+ of 500VDC unloaded. 470VDC + loaded with a pair of 6550's.
I don't want to believe the Drake 1202-324-2 PT is going bad. Can you think some thing i am missing here?
Standby switch is good and i have also removed the red leads from it to test the PT directly.

385VDC with 6550' wont get it.

How does the amp sound? 300VAC may be a bit low for a 50W amp, but 400VDC after the recto is right for 300VAC. Does the PT get hot? Put a pair of EL34s in it. Did the B+ increase? Is there anything wrong with the amp other than you feel the B+ should be higher? Maybe Marshall built a conservative amp for a change...

Maybe Marshall built a conservative amp for a change

I was thinking along those lines yesterday but have a hard time believing they would have put 6550's in this amp @ 385 plate top' on the power tubes (B+). I do not get any voltage increase using EL34 tubes either. The amp sounds like the bias is way off and the power tubes are serious cold and are not producing any real headroom to speak of.  Compared to what  you would expect from a marshall or even fender amp for that matter. The schemo is a mess with zero voltages. And on the internet as well i have found no concrete plate voltage B+ specs. One i found said they had a similar issue with about the same 385VDC that i am getting. But many others said there 4210's were so loud they could not turn them up past 4 on the dial. So it's been a real mixed bag of finding specifics on this amp's voltages.

PT is so big that i haven't noticed it getting hot at all. Cold would be more like it.

[sluckey]

What voltages do you have on plate, screen, grid, and cathode for both the output tubes? What is the bias cathode current set for for each type tube? You will have to adjust the bias to go from EL34s to 6550s.

[plexi50]

I think i already checked that and there was little difference between the 6650 & EL34 tubes plate and screen voltages.
I'll put in both sets of tubes tomorrow and make note of the plate,screen and cathode current and post the results.

[plexi50]

Ok! The PT info i got from MM says 360-0-360. That would put me @ 470VDC B+ before tube load. That's what i would be happy with.
So the PT is going south of the boarder. Don't see any other reasons to check the PT further?

[Ed_Chambley]

That is a Frank Levi modded amp.  He is one of the LA amp "gurus."  Do you know the mod(s) he did on this amp?  You might email him a franklevidesigns@gmail.com and ask him why such a low voltage reading.   Just a thought as I have never had any trouble with a Drake PT unless something damaged it.

You only get 300VAC when the HT wires are not connected to anything?

[plexi50]

You only get 300VAC when the HT wires are not connected to anything?

Yes that's it. Only 300VACunsoldered from the standby switch. Thanks for the info. I will email him and see if he did anything with the PT.
I have no other info on any mods that are apparent on the board.
I will open up the PT and take a peek to see if there are any resistors or anything else in there. Maybe there's a surprise inside?
Like a lost winning Lottery ticket!                                      

[plexi50]

I think i already checked that and there was little difference between the 6650 & EL34 tubes plate and screen voltages.
I'll put in both sets of tubes tomorrow and make note of the plate,screen and cathode current and post the results.

6650 pair @ 376VDC - Screens 372VDC - 47.5mv Grid
EL34 pair @ 397VDC - Screens 390VDC - 47.5mv Grid

This is a fast check. No real warmup time to speak of.

[sluckey]

I looked at the Marshall Reverb amp line number 180 in this forum's schematic library.

So did I. But I looked at the proper power supply/power amp.

[plexi50]

I see the 500 volt caps at the choke on the schematic so that pretty much tells me the PT must have experienced some damage at some point in it's life. Maybe it just  decided to retire.   

[Ritchie200]

I understand the 4210 is a 2205 in a combo (according to my Marshall book).  If C43 had a problem or was open that would put you at the voltages you are posting.  Maybe check connections on that first 50 cap.  I cant imagine one of those going bad. 


Jim

[Ritchie200]

Ooops sorry, I just saw the 300v unsoldered post - 


Jim

[plexi50]

Im ordering one monday. SHOULD HAVE DONE THAT YESTERDAY BUT THE WEEKEND IS HERE and it woudln't have shipped anyway. Man this dam laptop i have has a caps mind of it's own.

[Ritchie200]

One thing I did notice is D8 and D9 are missing and they have jumpers there.  According to my schematic you are down to single diodes on each side.  Not much protection there....


Jim

[plexi50]

One thing I did notice is D8 and D9 are missing and they have jumpers there.  According to my schematic you are down to single diodes on each side.  Not much protection there....


Jim

Can you post your schematic? I have found no layout diagram with part reference numbers. I was thinking the same thing having only (1) diode per side of the AC to DC rectification.. I have a new MM 360-0-360 transformer installed and wired up. I now have a healthy 450 VDC B+ under load with (2) 6550's. The reverb is output is pretty weak. The tank is good.                     

[Ritchie200]

Well, again, according to my Marshall book, the 4210 is the combo version of the 2205 head and I was looking at the schematic in the book.  I looked real quick in the library and all I found was that combination schematic that you cant read.  Here is a link to a clean one.  The Marshall book does have a few rare mistakes so I hope the reference is right!


Jim


http://www.kbapps.com/audio/schematics/tubeamps/marshall/images/2205STDpoweramp.gif

[Willabe]

I agree with Jim Bob, at that voltage you need at least 2x 1N4007 on each side, if not 3x 1N4007 each side so you have extra head room (voltage wise) for wall acv spikes.

At the price of modern 1N4007 SS diodes why not?

(Without opening a can of worms, I switched over to ultra fast 1N4007's, price by the 100 is very similar. UF diodes have a much faster turn on/turn off, so they help limit the 'switching' noise (transient spikes) by shortening the length of time of the 'turn on/turn off pulse' of SS rectification.) 
 

[plexi50]

Good idea for extra surge protection.
Now why do i have a weak reverb output? 300VDC on the reverb transformer. 680k reverb resistor in spec. All plate resistors good and FX working. Went through the foot switch jack and all looks good a far as the jack wiring goes. V4A & V4B is the reverb.
Have not checked the reverb capacitor's of V4. The reverb tank signal when connected is barley audible.
I am also going by the weak signal i am getting when i touch the reverb RCA output  tip. V4 reverb tube is good. BC184 at reverb pot?

Cathode voltages of V4A 2.116 VDC & V4B 1.006VDC.
Cathode voltages of V3A 1.892 VDC & V3B 1.175VDC

Think i have a bad wire connection under the board.  Moving V3A & V3B wires at board are causing a crackling breakup.
It is not the cathode wires as the voltage does not fluxuate or change when i make this occur. Sounds more like high voltage plate crackle.
V3 tube socket is good and not a bad pin connection problem. Interesting weird dam amp. Especially the channel switching  crap which has some bleed over on either channel used. I read the 4210 was Marshalls first attempt at channel switching.

[plexi50]

Well all the wiring is good and making connections on V2 & V4.
If i hold onto the reverb output tip it reacts and hums but fades out after a few seconds.

[plexi50]

I have since discovered that Marshall made (3) different Drake 1202-324 power transformers.
1202-324-1
1202-324-2
1202-324-3

Each transformers was made for different HT voltages.
The 1202-324-2 i have is 300-0-300 producing a B+ of 400VDC.

I thought it was odd a few years ago when i tested it and got the lower B+ voltage reading.
Perfectly fine Drake power transformer.

[Ritchie200]

AHAH!  Lotto ticket found!

Jim

[plexi50]

It's funny but at that time back in 2015 i did not see anything online about this transformers number sequence and the different voltages using the number 1202-324 and then the last number 1-2 or 3. Now there is info about this so i feel better knowing the transformer was not actually bad. Just had a much lower B+.