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HD130 head repair

Hi I was wondering if anyone had any tips, I have an HD130 head I bought in non working condition,(my 3rd HD130 in all!) it has no preamp tubes, and it came with a 4x KT77 tubes which I understand are supposed to be interchangeable with EL34

Repairs I've done so far:
* replaced power transistors w/ a matched 2N6488 pair
* replaced electrolytics on the power board
* replaced the LM1458 chip (these first 3 steps solved my other HD130 combo so I just went ahead and this to start with!!)
* replaced the 220R resistors on the tubes
* replaced the 1.5kohm 10W resistors.
* the largest electrolytic caps actually seemed fresh I think they are marked 2016 by whoever installed them

* hi power mode burns out the 3rd 220R resistor on the 3rd tube. eventually causes more issues if you keep it running, the 2nd pair of tubes starts redplating as eventually the 1.5kohm 10W resistor burns out as well, and then eventually itll burn out one of the power/bias transistors (I know all this unfortunately I was not watching the internals when I attempted to repair it so far!)

* Oddly enough it seems to work and sound perfect in low power mode.

thanks for any help!!!


Check Power Supplies.

Since the tubes are red plating, remove the tubes to safely do some static mode troubleshooting. Since there has been work done, go back and check the Lo voltage power supply zener diodes (D5,6 ) for correct voltages. Also check D17 zener diode for correct tube control grid bias voltage. Next check the Hi voltage supply outputs in both Lo (580V) and Hi (700V) Mode. Fix any out of spec voltages. Other problems will be easier to diagnose once all the power supplies are in spec. After that, reinstall the tubes for active mode troubleshooting. Good Luck. -mgriffin


Thanks, so much for the

Thanks, so much for the response, I'll give it all a shot and report back!


OK I finally got around to

OK I finally got around to some measurements:

D6: -16.62 V
D5: 16.89 V
D17: 16.67 V

As for the power, hopefully I measured this correctly, I took these from where the R16 & R67 (150k resistors) meet on the smallest board:
HI: 364 V
LO: 242 V

Still probing around for obvious defects, LO power mode seems to actually work unfortunately HI power mode with tubes will always burn the rightmost 220R

thanks again for any help!


at least one of the 1n4003

at least one of the 1n4003 diodes appear discolored, I wonder if that would be worth swapping out


Power Supply is OK

The HV Power Supply voltages look OK, both Hi and Lo mode. You measured at the junction of the R16,R67 voltage divider. The Zener voltages also look OK. None are shorted or open circuit. Not sure why one of the 220R control grid resistors fails when in Hi mode. Have you checked the 2 transistors that feed the tube cathodes? Can you set the transistor bias correctly with the TR1 pot? Follow this procedure to check and adjust. http://www.pacair.com/mmamps3/sites/default/files/docs/MM%20Service%20Bu... Use Section 1. since your amp does not use pre-amp tube.


First off, as always I just

First off, as always I just want thank you for the assistance, I truly appreciate it. I may share pics of my collection after I get this head up and running!

I am a little confused by the transistor bias trim pot calibration. Can this be done correctly without the tubes installed?

My multimeter seems to be testing continuity to ground with all relevant points, emitter and both sides of 3.9ohm resistors. I am not really getting any significant or consistent readings. Do I need a better multimeter or do I need to put the tubes in? I am willing to burn a few more 220R resistors I bought a couple dozen happen to be all matched values!



The tubes must be installed to do the bias adjustment. Since the amp runs OK in Lo Mode, do the bias adjustment in Lo Mode. It won't hurt anything. Also, measure the transistor collector voltages while it's idling. They should be between 60V and 90V. Post your results. Once the kinks are worked out, we'll tackle Hi Mode. Until then, we are trying to determine what causes the 220R grid resistor to fry in Hi Mode. Silly question: Could one or more tubes have been installed improperly? Sometimes the socket alignment tab malfunctions and the tubes don't go in correctly. -mgriffin


Alright I gave it a shot, I

Alright I gave it a shot! I had a good feeling so I put a fresh 220R resistor in. As I had it powered on in standby mode, The tubes seemed to be glowing healthily and evenly.

LO power mode:
I calibrated one transistor emitter to 25mV exactly
the other transistor emitter is reading 31.5mV

collector readings respectively:

I could see that the stripes on the 220R were already turning brown, so I figured eh might as well get a HI power reading , which might have been a mistake in retrospect, the resistor cooked up immediately so I only quickly glanced at some readings before I turned off power altogether.

I think the power voltages on the collectors might have climbed up to 41v and 46v and I think the emitters climbed up to 36mV and 41mV


Oh! yes and as for the tube

Oh! yes and as for the tube sockets, they are in correct orientation, I cleaned and reset the tension on the pins to make sure the tube was truly connected


Swap Around

This is confusing. I'm beginning to suspect a tube is failing. Have you tried swapping tubes around to see if the frying 220R resistor follows one particular tube? If yes, that tube may have some internal problems. If not, we'll keep scratching our heads. Thjs may be a tangent but I've noticed other MM Amps of this vintage (RD65, RP65, HD150) use a 22V zener feeding 220R for the control grids. For now, lets not go down that rabbit hole.


YES! That was the correct! I

YES! That was the correct! I swapped the 3rd tube to 4th position, and now the 4th position 220R is burning so it must be that single tube is failing.

I can't thank you enough for all the help thanks again!

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