Hot Rod Delux repair/mods

[beezerboy]

I got a HRD that has a blown output transformer. it also took out the two 470 ohm 1 watt resistors that current limit pin 4 on the 6L6s. I have the new resistors... the new trans is on the way. there is also 2 diodes in the circuit with the 470s. they are flyback protection, rated at 3kv. they test ok but they make nervous. closest I have on hand is rated at 2kv (which I hear is what Peavey uses)... any reservations about using those? I know guys also put 3 diodes in series to get the PIV up.... don't really want to do that. the Fender part is like 9 bucks, a Vishay equivalent is literally 25 cents. problem is shipping.... anybody know a source? Fender # 029690001.... though I'm really thinking the NTE 525 is how I'll go (2kv rated)

so, while I'm in there..... what else should I look for or do. I know about heat sinking R78 & 79 in the 16v power supply, and some have suggested a taper pot on the volume control... anything else? oh yea, new tubes for sure

[gbono]

Check all the solder joints on the PCB with the tube sockets..... Here is a replacement for the protection diode = Vishay GP02-30 (should be at Digikey).

[soundguruman]

The 3KV ones probably never go bad. That's why they use 3KV.
why you want to use lower voltage diode? That makes me more nervous, than using the original.
If there is no reverse leakage with your analog ohm meter when the diode is out of circuit, then use it, no need replace it.

It is true that the 2 resistors for 16V run hot, and gobble their own solder connections.
I donno if heat sinking the original is really the best way, I might try chassis mounting metal cased resistors
15 watts aluminium housed resistor | Heat sink resistor | Arcol resistors | Aluminum clad resistors
And I would use teflon 20 GA wire from the board...

you need to check replace the bias filter capacitor(s) if the amp is 8- 10+ years old...
this is a very cheap and critical part(s). you don't want that one to fail from age.

[beezerboy]

reason to use the 2kv diode is because I have them, and the 3kv ones are 3000 miles away. shipping to AK is like 30 bucks for UPS even though I live a couple miles from one of their biggest terminals. the circuit is a nominal 60v (output to the transformer). a relative "borrowed" my last analog meter years ago... 'spose I could run them up on a power supply & check for leaks.

copy that on the replacement resistors. good plan.


as for the biasing caps... don't see a traditional looking installation (maybe because I don't know what to look for) do you mean c 26/27? or possibly c40/41?

please forgive... I'm not really a tube tech.

[Enzo]

It is not a critical value. The 3kv and 2kv types both will do the job. They stay out of the way until you try to play without a speaker, then they squash the resulting spikes. Various companies chose various types for the same application.

Diodes and stuff like them weigh very little. UPS rates include distance. If you order small parts from some supplier, use the post office. Most have a USPS option. As far as I know, the post office doesn;t charge extra for AK.


The circuit those diodes are in is NOT 60v. The 60v on the schematic refers to the signal level at that point with the specified input signal, which is only 4mv. The signal level gets MUCH higher than that. But more than that, there is +431vDC there all the time.

C26,27 are coupling caps between the phase inverter stage and the power tubes. C40,41 are just filter caps on the 15v supplies.

The bias supply caps are C43 and C38, though why those particular ones would be more likely to fail than any of the other filter caps is beyond me.

[beezerboy]

silly me... I thought the majority of the B+ would be dropped across the transformer primary but ya, I think its a moot point.... never going to get close to 2kv under normal loads right?

any recommendations for replacement caps? (c43/&38) up on the voltage ratings? seems like I read some guys change the value too? (I ran into a mod somewhere on another board but don't remember where)

[Enzo]

Unless you have a good reason, just replace any parts you replace with the same thing they were. We just discussed that you have options for those HV diodes, but note none of the options change the operation of the amp.

As to changing things, everything occurs in a context. Guys don;t just up and change caps. Older fender amps often used 50v caps in 55v circuits. Right on the schematic it says 55v and 50v caps. They will survive that for a long time, but it is poor practice, so guys would use 100v caps instead. Your reservoir cap is already a 100v part. And 50 years ago, caps were more expensive than they are now - in relative terms. So they used as small a cap as they could. Guys would replace little 8uf or 16uf caps with something larger. Yours is already a 100uf cap. Your amp is 40 years younger than the ones they change cap values on.

There is almost no current draw on the bias supply, and that is mostly the voltage divider itself. That is less than half a milliamp. There will be no demands on it, so making the caps larger won;t do anything for you.

[soundguruman]

reason to use the 2kv diode is because I have them, and the 3kv ones are 3000 miles away. shipping to AK is like 30 bucks for UPS even though I live a couple miles from one of their biggest terminals. the circuit is a nominal 60v (output to the transformer). a relative "borrowed" my last analog meter years ago... 'spose I could run them up on a power supply & check for leaks.

copy that on the replacement resistors. good plan.


as for the biasing caps... don't see a traditional looking installation (maybe because I don't know what to look for) do you mean c 26/27? or possibly c40/41?

please forgive... I'm not really a tube tech.

Like I said, the diodes are not bad.
And why are you trying to re-design the amplifier with 2 KV diodes?
Fender put the 3KV in for a reason. Are you a better designer than Fender? Answer: NO.
And if the diodes read GOOD, why are you so obsessed with replacing them?

The bias filter caps are cheap Chinese (illinois capacitor) and they can fail.
When they fail, the output tubes turn cherry red and BLOW.
The investment to replace these, $1.59 is (believe me) well worth the money.


There are MANY models of HRD and you have to figure out which schematic matches the amp.
On my schematic here, 1995- 1996, C 43 is 100uF 100Volt, and C 38 is 22uF 63Volt. Those are the bias filters on MY schematic.
BUT what model / schematic do YOU have??? POST the schematic here.

You say you want to fix the amp, you say you want some advice.
You say you want to replace parts that you "think" need replacing, (even though they test good) but you are UN-willing to make the extra effort and get the RIGHT parts.
What's THAT all about?
No, No, No, you have not graduated to "amp fixer." LEARN to make the extra effort, spend more money, and do it RIGHT.
LISTEN to what the techs here are TELLING YOU!



"please forgive... I'm not really a tube tech." I can see that.

I (that's me) am a Certified Fender Amp Technician "FOR LIFE."
Don't listen to what I say, What the heck do I know???

[beezerboy]

I appreciate the advice all. I'm still gathering data. the diodes read good with a meter.... they may not be good under 400 volts. I'm also told that I don't need them at all.... I thought about testing them at voltage, but decided I'm going to replace them simply because they were in a circuit that had a melt down.

Although not a tube guy, I have been a tech for 40 years (aircraft), so what the heck do I know?. now relax and take a pill before your fuggn head explodes.

[Bruce / Mission Amps]

If you are worried about the stock diodes and you want to use the 2Kv diodes, see if you can strap a 5 watt 470 ohm resistor across the speaker jack leads somewhere in the chassis.
It's pretty unlikely that, with the 470 ohm load on the OT secondary and a speaker unplugged or blown open, the OT will ever generate enough flyback voltage to cause those 2Kv diodes to go bad.
Buy your parts from Mouser Electronics and choose the USPS Priority Mail option for shipping... you can get a 10lb box of parts shipped anywhere in the USA for $12.00 or less.

[soundguruman]

It's called OCD.

And yes, aircraft mechanics are intimately qualified to re-design Fender amps with underrated parts....

And since I know how to repair fender amps, I will start overhauling aircraft engines.....TOMORROW.

[Enzo]

There is no consensus. Various brands of amps use various flyback diodes, all on amps with the same power tube setup, and the same B+ voltage (480-500v). SOme companies use a 2000v part, some a 3000v part. I don;t see one blowing up any more or less often than the other. To blow the diode, you either have to get peaks that exceed the 2kv or 3kv voltage in reverse, OR you need to exceed the forward current rating of the diode. These diodes only have a 250ma rating. SO my money is not on the 2kv or 3kv being exceeded to kill these thing. My primary suspect is excess current.

[oc disorder]

"It's called OCD" ...someone mention my name ?

It's ironical this discussion on replacing the two diodes meant to protect the o/p transformer .. when they clearly didn't !

I had to replace 1 well replaced both .. recently in a HRD and the 470ohm screen resistors but luckily the o/p transformer was ok !

I wondered if it was really worth it but in this case the customer was happy to have the genuine Fender parts replaced.

Should anything go wrong in the future I can sleep at night knowing other "Sound Guru's" won't be badmouthing me
for being slack with failing to fit the designated parts!

Only cost me ten bucks Australian .. you guys have it easy...

Fender 0029690049 Diode Hv 3Kv 200Ma (6Pcs) | | orders@brookmays.com


All this waffle over a cheap bag of diodes....

[beezerboy]

so.... turns out I can't get the 2 500v caps I want to replace & would have had to do some other substitutions. I didn't post before but found some other caps that look like they are starting to leak, so the project grows. anyway, I placed an order with Mouser yesterday & includes the 3kv as well.... why not. (which were 40 cents). and actually...... Mouser didn't have the 500V caps either, so off to ebay where there are lots.

as for my professional experience, mr "guru"... ya, A&P, AI, pilot SEL, SES, MEL, I also have an AAS and a Bachelor Science, studied as an EE, and have probably 1000 hours of systems training (not maintenance) from Boeing & Airbus, etc and more.... so fuck you. you are a self aggrandizing snob full of you own worth. no point in replying to anything I post.... I'm not interested. just keep your distance

[Jazz P Bass]

Had to look that one up.

self-ag·gran·dize·ment (slf-grndz-mnt)
n.
The act or practice of enhancing or exaggerating one's own importance, power, or reputation.

Good description of SGM.