OT upgrade recommendation for SF Deluxe Reverb

[Tone Meister]

What does the "A" suffix designate with the Hammond transformers? If I'm gonna spend $100+ on an OT then you can bet I will choose one that will accommodate 6L6GC types. The plan for the moment is to install a pair of 6L6 and see how it affects the PT and proceed from there.

Performance wise, how do feel the 1650FA (7.6K) compares to the Allen TO-26 (7K) or even a correctly spec'd 1750 series Hammond, which seems to be 6.6K for the DR?

[Tone Meister]

I used this one for my Princeton/deluxe hybrid build. It handles 6L6's with no problems: http://www.classictone.net/40-18087.pdf

my work is directly across the street from where they are made...I walked over one day and talked to the engineer over there about my project. 40w should be no problem, it's overbuilt.

Thanks Ron, the ClassicTone is definitely on my short list but Pete has brought up the 6.6K primary thing. I want the option of running either 6V6 or 6L6 optimally, with nothing more than a tube swap and bias adjustment.

[pdf64]

'What does the "A" suffix designate with the Hammond transformers?'
It may signify that it's a modified version of another model. There's a 1650F, identical to the 1650FA except for the older complicated secondary arrangement Hammond Mfg. - "Classic" Push-Pull - Tube Output Transformers - (1608 - 1620, 1645 & 1650 Series)

I'm sure those other brand OTs are great but I wish they would publish more info, materials, performance characteristics etc.

FYI, my findings are that 6L6 in a GZ34 rectified Fender type output stage (420V static B+) put out the same power into 4k and 8k p-p (ie switching an 8 ohm load between 8 and 4 ohm secondary outputs).
The JTM45 uses 5881 / KT66 into an 8k p-p OT.
6L6 seem to give max power around 5 to 6k p-p.
The (massive) OT in DrZ EZG50 has a 5k5 primary.
A 70s Fender DR OT that I checked worked out at 6k2p-p.

Really, anything in the range 6k - 8k would be probably be fine.

[ron vogel]

Thanks Ron, the ClassicTone is definitely on my short list but Pete has brought up the 6.6K primary thing. I want the option of running either 6V6 or 6L6 optimally, with nothing more than a tube swap and bias adjustment.

Then you probably want a 6.6K...6L6 likes 4K, and 6V6 likes 8K (or less if voltage is over 400V). I can swap tubes with just a bias adjustment between the two styles. 6.6K is what the DR calls for stock. Either way, get a multi tap OT...will give you more options on impedance matching.

[The Dude]

Just "thinking out loud". If you do find that the PT can't do the job, you could just install a small filament transformer and save some $ as opposed to replacing the whole PT.

[Tone Meister]

Just "thinking out loud". If you do find that the PT can't do the job, you could just install a small filament transformer and save some $ as opposed to replacing the whole PT.

I have sure given a filament transformer some serious thought, Dude. Definitely the cheapest way out. I hope to experiment some and take some voltage measurements in the next day or two. But my imagination has been running wild and I may just get a wild hair and replace both transformers and be done with it, though. The ones I am considering are:

POWER
Mojo MOJO761
Primary 120 V, 60 Hz
Secondary 362 V @ 150 mA, with 52 V bias tap
Filament Winding 6.3 V, 4 A
Rectifier Filament 5 V, 4 A
~$59.76

-OR-

Allen TP40D
320-0-320 @180 mA
5V @ 3A
6.3V @4.5A
Internal hum shield brought out to a lead for grounding (ensures quietest amp possible)
~$95.00

OUTPUT
Allen TO-26
7K primary
8 or 16 ohms output
Made in the USA by Heyboer
3 1/8" mounting center
~$69.00

-OR-

Hammond 1650FA
30 Hz. to 30 Khz
7.6K primary
4-8-16 ohms output
~$105.00

-OR-

ClassicTone 40-18087
6.6K primary
4-8-16 ohms output
~$48.85

[pdf64]

'6V6 likes 8K (or less if voltage is over 400V)'

I don't follow your thinking, can you explain?
My understanding is that (as a general principle) as the voltage increases then (to keep plate dissipation reasonable) the load impedance has to increase too?

[Tone Meister]

Hope someone can clear this up for me. The BFDR schematic shows 320-0-320 and the SFDR schematics show 330-0-330 yet the MOJO761 shows 362V @150mA from the secondary. I called MOJO and they said those voltages were "consistent with the original." Once rectified, wouldn't that produce over 500VDC or am I missing something?

With the 6.3V @ 4A and 5V @ 4A the MOJO761 seems worth considering. What am I overlooking here?

[pdf64]

'362V @150mA'
I wonder if that's the unloaded voltage?
Yes, if at full load then it should rectify to 512Vdc, minus ~20V dropped across the GZ34!
And even higher under static conditions.
TBH I'm a little wary of Mojo PTs, after hassle with a 762EX that put 7.4V on the heaters. I had to add 1 ohm resistors in series to bring it within spec.
But then a 758EX in my BF type build works great, voltages as expected, and has served me very well.

[Tone Meister]

'362V @150mA'
I wonder if that's the unloaded voltage?
Yes, if at full load then it should rectify to 512Vdc, minus ~20V dropped across the GZ34!

Likely so, but the 362V @ 150 mA implies a load of 150 mA, or does it? I'd sure be a lot more comfortable if that info was clear in the documentation. The conversation with them was a bit too curt for my tastes, given it was a valid pre-sales question.

EDIT: MOJO says that 362V is the unloaded voltage and these are made for them by Heyboer.

[pdf64]

'362V @ 150 mA implies a load of 150 mA, or does it?'

Not so much as imply, rather it looks to be a direct statement.
I doubt that the 6.3V and 5V winding outputs are unloaded, so the spec condition may not even be consistent for all secondaries.
It shouldn't be this hard.
Whatever, at least it makes sense (kinda) now, thanks for the update.

[Tone Meister]

Here are some real world measurements.

Line voltage from variac: 120VAC

JJ 6V6 463V 19mA 8.7W
RCA 5V4GA 6.60V filament

JJ 6V6 470V 19mA 8.9W
China GZ34 6.60V filament

JJ 6V6 450V 19mA 8.5W
Tung Sol 5U4GB 6.60V filament

Winged C 6L6GC 455V 36mA 16.5W
China GZ34 6.56V filament

And for a much heavier PT load:
Winged C 6L6GC 426V 39mA 16.5W
Tung Sol 5U4GB 6.55V filament


Looks like I'm gonna start by upgrading the OT and seeing how the amp sounds at various voltages by swapping rectifiers.

[pdf64]

My take is that the 6L6 are running rather hot for that little PT, and hotter than should be needed to get rid of crossover distortion, given their 6k6 load.
Did you keep monitoring the variac output, especially for the 6L6 tests? I'm thinking that the greater current draw may have pulled the variac output down a little.
And the line here seems to vary a few V over time.

[Tone Meister]

16.5 watts isn't all that hot for a 6L6GC, especially a Winged C. The variac is a Staco 1520 and it stayed a very steady 120 VAC without any adjustments, so line voltage is a constant in these tests.

The PT for the 5U4-equipped DRs was rated at 330-0-330 and 3A for the filament. I don't mind changing the PT, but my inexperienced eye doesn't see an issue with these voltages.

What are you thinking Pete?

[Richard]

What other mods do you have planned for the amp? On my 68 Princeton (no reverb) I did the Stokes mod to the PI, went 40/20/20/20 for the filter caps, reworked the tone stack, and also raised the voltages in the preamp which probably did more than the the 6L6 swap. I didn't upgrade the OT so I'm sure that's a factor not getting so much out of the bigger tubes. I probably would have gotten more out of the tubes that way, but I was running into a 4 Ohm 2x12 cab. My goal was to make my amp as clean as possible ad it came out well. I don't know if the stock PI can really push 6L6's to full capacity.