Sunn model T output transformer choices

[GainFreak]

Hi,

I was wondering what would be a good choice for a Sunn model T output transformer build? I was considering the following Hammond transformers:

1/ Hammond 1650T 120W H-iFi. Seems like overkill though. The original one doesn't look that massive
2/ Hammond 1650R 100W Hi-Fi with mismatched secondaries for 2, 4 and 8 Ohms. Considering the 30Hz low limit it will take 150 Watts at 50 or 80Hz.
3/ Hammond 1750WR (013691) for Fender Bassman 135.

What do you think about that?

[Helmholtz]

What do you think about that?

I think a schematic would be helpful.

[loudthud]

Laminations for the original Model T output transformer are 4.400 x 5.256 at stack height of 1.766 (Z type mount). The Hammond 1650T laminations are 3.75 x 4.56, unknown stack height. My bass amp uses a 1650R on a Traynor YBA-3 chassis and power trans (similar to 378CX). Puts out 150W with 6550's.

The Hammond 1650R is about the same size as the output transformer used on 60W Sunn amps.

Note that the band SUNN likes Model T's, they tune their guitars down to C.

Link to schematic: https://el34world.com/charts/Schemat...nn_model_t.pdf

[loudthud]

2nd Gen schematic: https://el34world.com/charts/Schemat...el_t_super.pdf

[GainFreak]

Laminations for the original Model T output transformer are 4.400 x 5.256 at stack height of 1.766 (Z type mount).

Thanks for the info!

The Hammond 1650T laminations are 3.75 x 4.56, unknown stack height.

It looks like at least 2" thick which makes it close to model T stack size but I really doubt I'll need to go down to 30Hz at full power ever. AFAIK even an SVT OT doesn't go that low.

My bass amp uses a 1650R on a Traynor YBA-3 chassis and power trans (similar to 378CX). Puts out 150W with 6550's.

1650T is massive, maybe I'll consider an 1650R (mismatched) more closely since you confirmed it can output 150W.

[Mike K]

I worked on a project with a friend that was a bass amp with four 6550 tubes in a similar configuration to what Sunn used on many of their power amps. We used a custom Heyboer rated at 200W. This was long ago so I don't recall the core size or really any of the specs, but I could probably dig it up. It took a few months to get from them but the end result was good.

Merc makes transformers for those Sunn amps and we considered those but decided, at that time, cost was too high. Heyboer was still not cheap. We used a choke in the PS and I can't recall if we had that custom wound or if we used a Hammond. Anyway, it was a massive undertaking and wasn't without troubles. Initial build had some oscillations that had to be snuffed out and capping an amp running at 500+ V is a challenge in itself (you most likely will need to use series and parallel stacks to meet capacitance and voltage requirements). Plan on using a lot of space... The amp itself weighed a ton. We never measured the output as it would have taken a massive resistor to do so, but estimated it at around 160W of clean, tube power.


Also looking at the Bassman 135 schematic which 4x 6L6 in UL, I don't think you will get truly get the right performance at 150W out of any of the transformers. They claim 135 W RMS and rate the OT 40-15kHz. They are running those tubes at 500V loaded. In theory at 6550 should put out (and typically does) put out more wattage at that voltage and load (although Hammond states the UL taps are 20% vs 40%, so there is some difference there as well). My guess is it really will put out somewhere between 150-160W depending on your loaded B+ and bias. And my guess is that OT will not give you full 40-15kHz response at that power. I'm not sure how much you'll gain derating the 1650T to 40Hz, but unlikely it will be 30W.

[Mike K]

Heyboer_Choke.pdf

Heyboer_PT.pdf

Heyboer_OT.pdf


Here are the specifications for the iron we used for that build. Looks like our OT dimensions were larger than the stock Sunn (a little bit thicker stack). Perhaps we err'd on the side of caution. According to my notes we did have an idle B+ of 550V. So that thing was no joke.


Also I don't think we were off in sizing for 200W, I'm pretty sure I used this table when I spec'd that iron:

Push-Pull. Class AB1, Fixed Bias, Ultra-Linear Connection (40% Tapping Points):

Va,g2(b) ........................................................ ........................................................ ...................560 V
Va,g2(o) ........................................................ ........................................................ ...................553 V
I a+g2(max signal)................................................. ........................................................ .............2 x 157 mA
RL (a-a) ........................................................ ........................................................ ...................4.5 Kohms
Pout ........................................................ ........................................................ .........................100 W

So that's estimating 628mA at full signal for four tubes, output of 200W. I don't have our full load measurements but our PT spec was capable of this.