Mark IV power transformer running hot?? Any ideas

Hello,
This amp was just restored by myself I replaced the driver board with a VTA, Replaced the quad cap, Replaced the wiring, And lastly NEW tubes. I have two of theses amplifiers. One runs HOT to the touch. I have looked at both side by side and I have the quad caps installed correctly.The amplifiers look the same. Except one is running HOT. I swapped the tubes from one to the other no change? I'm lost could the quad cap be bad from the start? or could it be the transformer. How can I check the quad cap to rule it out and where should I look next

Thanks in advance

Tanner wrote:Hello,
This amp was just restored by myself I replaced the driver board with a VTA, Replaced the quad cap, Replaced the wiring, And lastly NEW tubes. I have two of theses amplifiers. One runs HOT to the touch. I have looked at both side by side and I have the quad caps installed correctly.The amplifiers look the same. Except one is running HOT. I swapped the tubes from one to the other no change? I'm lost could the quad cap be bad from the start? or could it be the transformer. How can I check the quad cap to rule it out and where should I look next

Thanks in advance

Does it get Hot really fast,or is it over a period of time? I have two newly built Mark IV's that have PT's that get very,very warm almost hot after a period of time. I don't think it's the quad caps,if there were a short,or heavy leakage, your fuse would probably blow. If you had a weak cap,you would have a nasty hum!!

I have a very slight hum on this amp, None on the other one. Are you thinking the cap could be bad? The hum is a very low level and it could be from the driver tubes, there old RCA clear tops 12au7's

One thing to ad The amplifiers sound great really nice with no issues other the the heat from one. It takes 10-15 before it gets hot but you cant hold your hand on the power transformer its to warm. I noticed the wax from the choke gets warm as well almost a drip but not quite

What you can do is remove all tubes from both amps except the recitfier. Then turn both on, and measure the open circuit voltage across the quad caps of both amps to see if there is any difference. Allow them to run for at least 30 minutes and see if the transforemr still gets hot. With no load from the rest of the amp, it could only be the transformer itself (shorted windings), the rectifier, or the quad cap.

Hope this helps,

Tony

Ok Ill do that and report back Give me a bit

OK I have 545v going to the VTA board on both amps, The voltages are within 1-2 volts of identical. The transformers are about the same temp,just warm to the touch but not hot.. Thinking back I was just playing the one for 20-30min then I ran both as a pair for 10 min and noticed the first amp was warmer than the second. Maybe they are working fine? I shall play them for an hour or so and get a temp reading on both. I was shocked at how hot the transformer got compared to my ST-70. Do the MarkIV's run HOT by design??

Tanner wrote:One thing to ad The amplifiers sound great really nice with no issues other the the heat from one. It takes 10-15 before it gets hot but you cant hold your hand on the power transformer its to warm. I noticed the wax from the choke gets warm as well almost a drip but not quite

Are you using the 5AR4 rectifier? I do know that some people have used the 5U4 types which draw larger filament currents.

Yes I am running EH 5ar4's I also did the diode mod that is mentioned on the board. There is a sticky post at or near the top

Are they biased the same? How about the output tube cathode resistor, same value on both amps? Can you measure the AC amps on the primary side to see if one amp pulls more current than the other?

Sal

Ok guy's I did some more research and did a few other things. I installed thermisters on the power line going in to the power transformer and it lowered the voltage from 121vac to 114vac so I was happy with that improvement. Next I adjusted the bias to .40v on all four output tubes. Then I just turned them on and let the amps play. Yes they get HOT but they are drawing the same amps and not blowing fuses. I guess its the nature of the beast to run hot or just very warm. Both amplifiers sound AMAZING . If its not broke don't fix it.

Thanks for the advise on what to look for on the MK IV's Ok now I need to get a fan LOL

And I want to give a very special thanks to Kevin a Dynakitparts for all the help as well

I hope that is a typo on post #7. If you have 545vdc going to the VTA driver board, something will blow soon.
The board is designed to run on 400vdc. There should be a 6.8K resistor between the main B+ and the board that would drop about 80v
from the B+, which should only be around 480vdc when the amp is running with all tubes in it.
Maybe you checked that with no tubes at all, thus no current and no voltage drops anywhere.

The voltage was checked with just the rectifier tube and the other tubes were out. The voltage was 510-518. Once the other tubes were installed the bias was perfect with the pots almost dead center. I think all is well

I installed thermisters on the power line going in to the power transformer and it lowered the voltage from 121vac to 114vac

That's quite a drop there , Tanner, how did you do it? Did you have to use a few of them? is one enough?

The main here in Montreal is 123 Volts in summer. My st-70 can deal with it i'm sure, since it's a kit I got from Bob with the new Power Transformer, but i could certainly use the infos for my pair of Quad II that has power transfos that were designed for 115-117 Volts.

Thanx for your help.

Yves

The thermistor I used are NTC CL-80 I purchased them on E-bay. I put one on each power wire going to the power transformer

Ok I have more bad news about the MK IV's I have restored. First off there is a POP heard thought the speakers 6-7 seconds after the power switch is pushed to the On position. Second I get oscillation sometimes though the speaker about 60hz. I have purchase 2 sets of power tubes, and MANY driver tubes. The voltage going to the board is a solid 414-408 never higher. The quad caps were replaced as well as the driver boards with VTA ones. The quads got pulled and shipped back to Dynakitparts to be tested and there are no problems with them. I have exhusted my time and effort on these amp's. What more can I do?? I have looked at them for hours and tested ever G-D damm part I can test. I cant get them to work correctly. The tubes like I said 2 full sets, plus I pulled tubes from my working ST-70 and still the same stuff. Popping and then they will play GREAT, and then they go to shit?? 60hz oscillation and I have to power them off. I'm ready to give up. I will not sell them to anyone because of the issues. Trash day is on tuesday every week. When the quad caps get back. I will reinstall them and yet one more set of tubes. If the issues are still there TRASH they go. I have $400 in the pair to start $350 in chassis and hardware from Dynakitparts. $150 in tubes, And $35 in paint and misc items. I'm sorry to rant but If anyone has any ideas please let me know, as these amplifiers are on DEATH ROW

Just to ad this. The issues are with the popping are with one amp, the oscillation happens with both?? The tubes are Svetlana EL34's. Electro Harmonic's 6CA7's, and a set of vintage Mullard EL34's. The drivers are RCA's and Conn's and Electro harmonic's.

Thanks in advance for any ideas on whats going wrong with them. I have been playing with electronics for 20+ years and can solder perfectly, as well as diag most amplifiers. I have restored 2 ST-70's as well as built a preamp from a kit (it was not easy with limited information) But I got it up and working after many hours of working with it. These amp are really leaving me with a bad taste in my mouth!!! . I am kind of new to vacuum tubes so I am quit sure I am missing something. I will not trust these amps to power my main speakers because I am scared they will mess up and damage them. I have played them only though my test speakers on the bench.

Could on of the chokes be bad?? what will a bad choke do??

Did you ever verify proper operation with the stock boards, before replacing with the VTA ones?

This is something I always do when working on an amplifier. Even if I plan to upgrade it with
new boards, I make sure it works in stock form, first.

Well I feel I must tell everyone I have the MK IV's working correctly. I had the cap can wired incorrectly, and the input wires from the driver tubes to the output tubes out of phase. I think this caused the issues I was having. The power transformers are running much cooler and the amplifiers sound PERFECT. I just needed to take a step back and look at the circuit at a later time, and I saw the issue instantly. It was a 5 min fix . Thanks to all that helped me and to those who listened to my bitching about the MK IV's. I cant thanks you enough !!

Tanner,
I was feeling your pain reading through each of your posts...

It's good to hear you have solved the issues with both of these amps.
Thanks for the update! Enjoy!

Regards...Kevin

Could you post a shot of where you wired the thermistors under the hood of your MK IVs? Is it wired in series on the power cord?