Hammond's Vibro-King PT has an Odd current rating

[bob p]

Now this is really odd --

Hammond makes replacement PT iron for the Fender Vibro-King. The unit is the Hammond 290SX. What really strikes me as odd are the PT ratings:

330 VCT @ 1.5A
53 V @ 0.75A
6.3V @ 3.75A

http://hammondmfg.com/pdf/EDB290SX.pdf

The V-K is a 2x6L6 amp with B+ = 465V. It has a built in reverb unit that runs a single 6V6 in Class A, and a total of (5) 12AX7.

Can anyone explain why the Hammond iron would have a 1.5A secondary rating on the B+ winding? I'm wondering if it's a misprint.

Schematic attached.vibro_king_service_manual.pdf

[bob p]

for comparison, their Twin Reverb PT is rated at 518mA which is only 1/3 of the VK current rating:

http://hammondmfg.com/pdf/EDB290FX.pdf

[Dave H]

It looks like the first one is 330V CT and the second one 640V CT if I'm reading it correctly but 330V CT can't be right can it?

EDIT
It is right. I checked the schematic. The Vibro King has a bridge rectifier across the whole 330V winding. The centre tap isn't used.

[Tom Phillips]

Hammond may have made their model 290 SX for some other purpose and then realized that they could Market it as a vibro king replacement also.

[g1]

There are 2 versions of Vibro-King in that manual. The first uses 6BQ5 reverb driver and I think the parts list shown is for it.
PT is listed as 026554.
This is also the export PT for red-knob "The Twin" and a few other 100W amps.
The Hammond 290SX references 026554.

The Vibro-King with 6V6 reverb driver shows 047777 as the PT. It has voltage doubler bias supply, so I don't think the 290SX works for these.

Probably still doesn't answer your question.

[bob p]

It's time for my lunchtime bowl of Alphabet Soup:

Schematic revisions E and F use the 6v6 reverb driver and list the 047777 PT number on the schematic. This is an export version of the transformer and it does not seem to be available in the N American supply chain. If it is then I could not find it from the suppliers I checked.

Schematic revision A uses the 6BQ5 driver, but the schematic does not appear to directly call out the PT by number. (If it does I somehow missed seeing it.) I do see the parts list referring to both 6BQ5 and to 022554, so I think you're right -- the parts list was created for Revision A and probably has not been updated, and if that's the case then it's safe to infer that 022554 is the PT for Revision A. But I don't see any PT number on the Rev A schematic.

Thanks for pointing out that 022554 is the export PT for the red knob twin and other amps. I was unaware of that. That's good to know, as it should mean that the PT is available in the supply chain, but I can't seem to find anyone that stocks it either.

I also see that you're right -- the Hammond site references 026554 for the vibroking on this page:

https://www.hammfg.com/electronics/t...rs/classic/290

But I can't find any other references to 026554 so I'm guessing that has to be a typo.



You're right again -- none of this answers my original question. I still have no idea why the Hammond 290 would spec power for what seems to be a ridiculous 1.5 amps on the B+ winding.

What's odd is that none of those Fender PT we've referenced seem to be available. As far as sourcing a PT for a build is concerned, I'm thinking that the only option might be to look for a something that could handle both a pair of 6L6 and the 6V6. that would seem to call for an 85W-100W PT and adapt the circuit to use the 6v6 version of the amp and whatever bias/power supply circuits will work with that PT.

I'm guessing that they chose to use a 100W PT because it was an easy off-the-shelf solution that could handle the load of 2 6L6 and a 6V6. Unfortunately that type of PT puts the B+ in the range of a SFTR, which is a bit higher than I would have liked.

[g1]

The non-export version PT for the red knob twin was 026477. Also used in Zinky's Dual Professional and Tone Master.

[cjenrick]

draftsman working late, slipped decimal point, s/b 0.15 A or 150 ma. (49.5 VA instead of 495 VA)

53 volt wind is also screwed up, s/b 0.075 not 0.75 Amps (4 VA instead of 40 VA)

6.3 volt current is ok , (24 VA)

from the dims on the spec sheet we see they are using a 125 EI core which is good for about 80 VA (sq stack)

we have 50 + 24 + 4 VA = 78 VA

running right on the edge, unless they ship a 1.75 in stack which is good for 110 VA.

[bob p]

fender used twin-type transformers in these amps to accommodate a 60W output section along with a champ added on.

150mA can't even be close to meeting this amp's needs, can it?

[Dave H]

150mA can't even be close to meeting this amp's needs, can it?

The 55W example in the 6L6 data sheet says 210mA plate and 22mA screen so it has to be over 250mA DC for the whole amp. The transformer rms could be twice that with a bridge rectifier. I try it in LTSpice.

[bob p]

we can't forget about that 6V6 that's running in Class A.

[Dave H]

LTSpice says 550mA rms.

What's the 6V6 current? I'll add it and run the sim again.

[g1]

The TAD equivalent of 047777 (6V6 reverb version) specs 600mA.
Mains Transf. for Fender Vibroking,Tonemaster - Mains Transf. for

[bob p]

I saw that TAD transformer. When I saw the price of 200 Euros plus shipping from Germany to USA, and probably an import tariff, I stopped reading. As a result I completely missed the PT rating of 600mA on the HT winding. Thanks for posting that.

As a conservative guess, I'd say try 50 mA for the 6v6 and preamp tubes.

[Dave H]

I saw that TAD transformer. When I saw the price of 200 Euros plus shipping from Germany to USA, and probably an import tariff, I stopped reading. As a result I completely missed the PT rating of 600mA on the HT winding. Thanks for posting that.

As a conservative guess, I'd say try 50 mA for the 6v6 and preamp tubes.

Plate current + screen current + 50mA is about 280mA DC which is 608mA rms for the HT winding according to LTSpice. That 600mA rating looks spot on. There's certainly no need for 1.5A