Dual Output Transformers

[jon]

What are the advantages and disadvantages to running dual OT's on an amp. I know Marshall did this for a while on the JTM45/100.

[Phil_S]

As an amateur, I don't feel comfortable speaking on the technical aspects of dual OTs. On the practical side of things, I can't see a good reason for doing it. It's more expensive, it takes up a lot of space on the chassis, it's probably heavier, and I can't understand what it accomplishes that you can't do with a single OT (except maybe stereo). It seems to violate the KISS rule being overly complicated and for what?

If you wanted multiple impedance options, primary or secondary, you could just wind and tap, as appropriate, a single OT. It might make for a more expensive OT to add taps and some additional winding, but I can't imagine it would cost as much as two OTs.

[Firestorm]

According to the Doyle book, Marshall only did it early on because they couldn't get transformers capable of handling the power of four output tubes. It works in terms of dividing power between the two, but also changes the inductance, so two OTs won't sound the same as a single, higher-rated OT.

As Phil says, there's really no reason to do it nowadays in push-pull circuits, unless you're trying to replicate a specific sound (as Marshall did in some JTM100 reissues.

There may still be an appropriate application in SE since high-power SE transformers are very expensive and kind of hi-fi sounding.

[roberto]

+1 for Firestorm. Marshall had no 100W trafo ready for that time (they used RS trafos IIRC), so used the simply equation: double the power, double the tubes and double the OTs.

[lumox0013]

warren hayes favorite amp is his dias with dual ots Ive often wondered if the dual ot thing is worth a try. I love warren I saw him when he came through last time for man in motion ,serious sound mostly the dias and a PRS.

[FUCHSAUDIO]

Warren also favors unmatched power tubes in his amps (based on my discussions with his tech..)

[Cliff Schecht]

Warren also favors unmatched power tubes in his amps (based on my discussions with his tech..)

I'm the same way. Some people hear the difference, others don't. I like to think it makes my Fender clones *somewhat* closer to the originals.

[rdjones]

+1 for Firestorm. Marshall had no 100W trafo ready for that time (they used RS trafos IIRC), so used the simply equation: double the power, double the tubes and double the OTs.

My understanding is the same regarding the 100W Univox with 2 OTs.
These amps require 2 speakers to be connected at all times.

rd

[TooL46_2]

Weren't the OTs connected in series? HT on the connected end of OT1 and beginning of OT2?

Can anyone spare a schematic how did this look like? Thanks!

[renshen1957]

Weren't the OTs connected in series? HT on the connected end of OT1 and beginning of OT2?

Can anyone spare a schematic how did this look like? Thanks!

Hi,

I don't have a Univox schematic, however Garnet amps had a RAM Line and Private Label Lines (made for department stores) beginner model that had two SE OTs in parallel called the 10P. The amp had two 50C5 power tubes and a 12AX7. Heaters were in series with line voltage as no power transformer. One of those mains voltage powered amp that a caution to work on. Amp produced 4 watts RMS.

Gar Gillies book on Tube amps has two illustrations on page 77: a Garnet Deputy (4 power tubes) with one 8 ohm speaker connected to 2 parallel PP OT on the 4 ohm taps, and another with 2 tubes powered by 2 OT parallel each with its own separate speaker but powered only by 1 B+ connection.

Scanners down or I would scan.

Best Regards,

Steve

[TooL46_2]

Thanks Steve! If you scan those pages by any chance -- I'd love to see them.

NB: running the UNIVOX amp straight from the line (without PT) sounds very risky...

[rdjones]

OOps, I said Univox but it was a Silvertone I was thinking of.

http://www.freeinfosociety.com/electron ... ne1485.pdf

The outputs operate separately from each other, like a dual mono doubled version of the 50W circuit.

rd

[renshen1957]

Thanks Steve! If you scan those pages by any chance -- I'd love to see them.

NB: running the UNIVOX amp straight from the line (without PT) sounds very risky...

Hi,

I will have the scanner fixed by tomorrow, hopefully, its one of those scanner, xerox, fax, printer combos by HP. Run out of one color ink cartridge and then nothing works.

Univox, Garnet, Silvertone, etc. budget lines took a page from the American 5 AM tube radios.

For those not acquainted, the cost cutting involved eliminating the power transformer and running rectifying 110V AC mains voltage and after the rectifier's voltage drop there was circa 110V DC on the plates screens tubes like the 50L6 (octal) or later seven pin tubes (briefly the 50B5, replaced by the 50C5). The later series string consisted of 35W4 (halfwave rectifier), 50C5 PT and plus 3 other radio tubes so the string equaled line voltage AC (adjusted by a resistor) for the tube heaters (all had the same 150 mA heater current in the series string). I believed some of these Radios were UL listed.

The tube compliment for is usually the 35W4, a 50C5, and 12ax7 tube. The SE transformers were as small as could be made without incurring extra expense for miniaturization. Think a minimalist Tweed champ on the very Cheap. Volume pot had the off on switch. The amps put out a staggering 2W of power.

The budget or beginners amps I've worked on are a potential electrocution waiting to happen, only a 47nf cap 600VDC (wax paper and foil) is between the electric mains and the chassis ground.
I insist on the installation of a 120 to 120 isolation transformer on repairs, along with a three prong plug. If the owner insists on keeping the amp original, then I ask for a signed waiver for release of liability.

Best Regards,

Steve

PS The same can be said for many for old tube portable record players as to minimalist designs.

[renshen1957]

Thanks Steve! If you scan those pages by any chance -- I'd love to see them.

NB: running the UNIVOX amp straight from the line (without PT) sounds very risky...

Hi

Please see attached files. Files post for commentary under fair copyright use. If you are thinking of building Power Transformerless design from the schematic, for everyone's safety, install either a 120V to 120V transformer or use two 120V to 6.3V wired with the secondaries connected as 120V/6.3V to 6.3V/120 as an isolation transformer.

The two pages are from the How and Why of Guitar Tube Amps as "Gar" Sees it by Gar Gillies of Garnet amp fame. http://www.garnetamps.com/book.htm

For $57.00 USD there are over 100 schematics besides a wealth of information and Ideas from a pioneer in the industry in Canada. If you are into the Guess Who or BTO sound Garnet Amps were in a number of their songs, including the Herzog on American Woman.

Please see attached files. Files post for commentary under fair copyright use.

Best Regards,

Steve

[TooL46_2]

The connections as in model 10p is how I understood it. Thanks for the scans.

Just to be clear -- I'm not planning on building any power transformerless amps at any stage. Nor would I ever recommend to anyone building such. I know the risks and would never do this