Peavey Rockmaster preamp power transformer

[joedonchev]

Hi guys,

I have a Peavey Rockmaster rack tube preamp.

The power transformer burned long ago and i had it rewired without specs - the guy just counted the turns and made the same amount of turns with a new wire.
The result was a transformer with secondaries 240V and 22-0-22V
So, the preamp was up and running again, but just for about an hour before it burned again - overheated.
I assumed that the rewired transformer didn't have the exact specs as the original, but i couldn't find info anywhere, so i just left it isle for years.

Now, i got interest in learning about tubes and tube preamps and I came back to that Rockmaster, wishing to bring it back to life.
I just could not understand how 22-0-22V can work with the filaments and keep wondering what might be the right specs for that transformer.
I just knew about the typical 6.3V AC filaments or 12.6V DC filaments, but nothing about 22V..
I found some clues by Enzo, who suggested that the tubes (4) are running in series/parallel and indeed need 24V DC supply and the filaments supply was used for the relays as well.

Firstly, i contacted the transformer manufacturer - "Basler Electric Co" with the part number 705-29303 (that's the 220V mains version), but they said only Peavey can give me the specs (it's their property).
So, i contacted Peavey with the same question, but they never replied.

Finally, i hope you guys can help me get as close as possible to the original transformer specs.

[glebert]

If you call Peavey tech support during their business hours they will usually give you whatever info they have.

[Enzo]

Yes, CALL Peavey support.

Oh, you are in China...

the email to use is customerservice@peavey.com

[joedonchev]

Ok, I'll try that. Thank you!

[glebert]

There is an old-time tech that I usually talk to at Peavey and has always been really helpful. If you don't get response to your email I can send you his direct email address.

[joedonchev]

Oh, thank you so much!

Before i wrote to repair@peavey.com. They haven't replied yet (2 weeks now).
Now i wrote to customerservice@peavey.com, as suggested by Enzo.
I will give it some time. If I don't get a reply I guess I'll just have to call them on the phone.
If nothing works, then I might need to ask you for that contact. Highly appreciated!

[cjenrick]

i might have rewound one of those, the trick is that it has a slim profile to fit in the 1 U case, so it might be hard to get a regular transformer to fit, i do not think the plate voltage is that high as it is a preamp, heaters are series parallel, divide DC volts by about 1.3 to get secondary volts as regulation tapers off with small lams, big transformer for twin reverb maybe divide by 1.4,

popular mod is to replace 50 k pot with 1 meg so you have two ultra crunch channels instead of one, problem is, PV no longer carries the pot with the switch, so what i did was use a regular pot and put a micro switch on the front panel, this allows a good Alpha pot instead of the crap pot which is stock,

schematic is horrible, took me a week to redraw it on 2 x 3 foot piece of paper,

here is estimate of pwr xfmr that will get you there, good luck!>

[J M Fahey]

Fully agree that schematic is ugly, poorly drawn amd misleading.

1) it shows transformer as only having *one* secondary winding: the HV one feeding 4 x 1N4007 diodes into a 47uF x 500V cap, which makes me think that the 220V you had on your rewound transformer are correct , for a raw initial 300V +B, typical of "starting" voltage in preamp supplies.
You can only go down afterwards.

Above suggested 120V secondary sounds low to me, and wouldnīt justify the relatively expensive and hard to find 500V caps when cheaper easier 250V to 350V ones would have been ample but of course I may be wrong.

Yes, tubes can work with 120/150V supply, but that scratches me the wrong way when talking high gain preamps which extract up to the last drop of gain.

Funny no LV winding is shown on PT and is labelled as coming from "secondary transformer" instead of "transformer secondary" , not the same thing.

*Maybe* schematic is from an earlier version? , where they split the job in two : a 220V secondary one and again a 22+22V one? ..... weird.

I would slightly modify the LV supply, turning it into a bridge rectified one (adding 2 diodes) instead of the current "full wave" one, which allows for a single 22V *thicker wire* secondary to work somewhat more efficiently.

Filament supply is cheesy poorly drawn and incomplete, but I guess raw ~30V supply is padded to ~24V through R300 (4.7 ohm, 1W) and feeding series connected 24V filament strings.

After repairing preamp, check that each 12AX7 gets about 12V DC across pins 4-5 .

I bet you got a marginal transformer in yours, and a turn for turn replacement wonīt work any better, I suggest you try getting a same HV winding but slightly reinforced 22V winding, since itīs the one doing the harder job: filament supply.

Sadly space is very tight and limited inside that 1U high rack.

2) Plan B: absolute worst case, remove tightly fitting internal transformer, add a suitable socket to chassis , and make an external supply whatever physical size is needed for reliability, and feed preamp +V and +30V DC from "outside".

Some Classic tube amplifiers did that: a power amp and supply at the bottom of the cabinet and a preamp on top, joined by multi wire cable harness and , way back then, octal connectors.

Or, since you must be using that cool preamp with some external power amp, maybe you can mount the extra power supply there.

3) Plan C: rewind that internal transformer with thicker wire) so it *only* deals with HV (so you donīt have an external cable dealing with +300V) , plus a simpler connector and feed it 22 (or 22+22) VAC from an external transformer or a tap/winding in the main power amp PT.

[joedonchev]

Ok guys,

Finally!!
I've got an answer from Peavey:

The secondary specs for the 70529303 are:
Sec 1 322 vac @ .041 amps
Sec 2 41.9 vac CT @ .403

Thank you Enzo for pointing me to the right email address! )

My hottest question now is:
"41.9 vac CT" means 21-0-21V ??

Second question is:
Should I request 1A for the "Sec 1" and 1 or 2A for the "Sec 2" when making the new transformer?

Meanwhile, i found that another Peavey rack preamp is using the same transformer (same part number) - Peavey TB Raxx
And I've got clean and complete schematics of both the Rackmaster and the TB Raxx:

https://www.dropbox.com/s/3949a6tby5...ock_master.pdf
https://www.dropbox.com/s/6xb8tbodvs...0TB%20Raxx.pdf

I hope no one ever need to know this info, but if happen, here it is.

[joedonchev]

Sadly space is very tight and limited inside that 1U high rack.

Yes, the original transformer was quite "custom" shaped and the new one won't fit the 1U height, as there's no such standard available, but at least it can still fit by width and length at the bottom of the enclosure.
It will get ugly, but I will definitely need to cut the top cover of the enclosure.
The plan is to make an external wooden box for it (making it look like a slim "head" with about 2U height) with openings at the bottom and the back (for ventilation) and a small fan on the back, powered by additional mini 5VDC supply, or if I find a 12V fan and manage to get a line from the preamp's relays supply would be better, I guess.

About the caps - I'm currently living in China, so many components are easily accessible here Even the German ones, but with some crazy prices.

[cjenrick]

here is a good website on the PV>

http://snw.lonningdal.no/peavey/rm-tctp/

here is a schematic that came out worse than the original

[joedonchev]

It seems I sent the "Rock Master" amp schematic instead of the "Rockmaster Preamp".
Here is the real one:
https://www.dropbox.com/s/de7nyu1o01...reamp.pdf?dl=0
Same i got from the Peavey's customer support

[J M Fahey]

Canīt download it, suggest you straight attach the PDF here.

Click "go advanced" and it will show needed attach file buttons.

[joedonchev]

Yes, but it seems i have only 976.6 KB limit for uploads, which i already exceeded by 200k in the beginning of this post.
The PDF is 1.3 MB

[Jazz P Bass]

peavey_rockmaster_preamp.pdf