# What Can I Build With These Transformers?



## GreasyB in YYC (Jan 6, 2020)

Several years ago, I bought Heyboer replacement power and output transformers for my Marshall JMP 2203 100W.

I didn't end up installing them and they've been collecting dust ever since.

My question is:

What sort of amp can I build using these transformers that isn't a 100W amp?

Would they work for a 20 to 50W build?

Would they work in a bass amp circuit?

Does anyone have any other creative ideas to put these transformers to use?

Thanks in advance!


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## Alan Small (Dec 30, 2019)

this is a Paul question...surely he will have some excellent ideas


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## Paul Running (Apr 12, 2020)

Once the TFM is characterised, the application becomes predictable. If you can obtain the detailed specs on the TFM it would help. I usually test all my OPTFMs with an audio generator and a scope. I send 10,000mVp-p into the primary (plate-plate for push-pull), then measure the secondary output. A voltage ratio can then be calculated. To calculate the impedance ratio, square the voltage ratio. Example from my inventory:

*OPTFM-23317*
Eprim = 10,000mVP-P
Esec wht = 240mVP-P, brn = 340mVP-P, blu = 470mVP-P
ERatio wht 41.7:1, brn 29.4:1, blu 21.3:1
ZRatio wht 1739:1, brn 864:1, blu 454:1
4Ω load requires 7KΩ, 3K5Ω and1K8Ω plate-plate load.
8Ω load requires 13K9Ω, 6K91Ω and 3K63Ω plate-plate load.

A typical 100W OPTFM will be challenging to distort at low power levels...it will require some creative thinking to fool the TFM...but it could be done.
There are a number of options with your OPTFM, limited by the mass of the TFM, gauge of wire used in the windings, type of core and several other factors. For a Bass application, probably 20W...depending on the frequency response.


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## Paul Running (Apr 12, 2020)

First 3 chapters of this book apply:


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## Paul Running (Apr 12, 2020)

The Mojo site uses Heyboer TFMs, as spec'd:










*OPTFM 100W-800 *(plate-plate = 1K8Ω)
Zratio Grn = 112.5:1, Yel = 225:1, Blk = 450:1
8Ω load connected to Blk would require 3K6Ω plate to plate impedance...for a 50W amp use 2 X EL34 in class AB1 or 6L6GC in class AB2, as shown below:


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## 1SweetRide (Oct 25, 2016)

Disappointed, came in to see this.


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## Jerome (Dec 4, 2015)

Well you could build a 100 W bass amp with them , or a 50 W amp , but they are not good for a 20 W amp , though you could come up with a design with EL34's in triode mode perhaps . All depends on how much experience you have building electronic projects . Might be best to sell them otherwise .


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## GreasyB in YYC (Jan 6, 2020)

1SweetRide said:


> Disappointed, came in to see this.
> View attachment 347713


🤣🤣🤣


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## GreasyB in YYC (Jan 6, 2020)

Here is all the info on these transformers that I could find.


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## Paul Running (Apr 12, 2020)

The specs on the PTFM are very good...lot's of potential for that iron. Bridge rectify the 370VAC for 520VP to run the plates of the output tubes; also bridge rectify the 304VAC, choke input that for 270VDC to run the screens of the output tubes and further RC down the line to the early preamp stages. Run the PI-driver from the high B+.
The Blu is 102VAC. I would also bridge that and use for your C-supply...bridge modules are very affordable these days.

The OPTFM that you posted, is a Hammond 1750U:
It is closely spec'd to the Mojo branded Heyboer.
You could verify DCR with a good DVM, use this chart from the 1750U:


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## GreasyB in YYC (Jan 6, 2020)

Paul Running said:


> The specs on the PTFM are very good...lot's of potential for that iron. Bridge rectify the 370VAC for 520VP to run the plates of the output tubes; also bridge rectify the 304VAC, choke input that for 270VDC to run the screens of the output tubes and further RC down the line to the early preamp stages. Run the PI-driver from the high B+.
> The Blu is 102VAC. I would also bridge that and use for your C-supply...bridge modules are very affordable these days.
> 
> The OPTFM that you posted, is a Hammond 1750U:
> ...


I was kind of hoping to incorporate a tube rectifier, because all my builds have used a bridge rectifier and I wanted to try something different.

What I'm interpreting from everyone's ideas is that the PT could work in almost anything, depending on how the B+ is managed.

However, the limitations to build variety result from the OT. Is that correct? Depending on speaker values and wiring configuration, could this OT be used in a Tweed Deluxe, for example?


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## tomee2 (Feb 27, 2017)

I think rob robinette has something to say about converting a twin reverb to use 6v6s so maybe 40 watts instead of 80. Or about using only 2 tubes instead of 4 but changing the speaker tap.
Similar problem here but with 4 EL34s instead of 6L6s.


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## tomee2 (Feb 27, 2017)

Amp Mods


Generic Guitar Tube Amplifier Modifications



robrobinette.com





Took me a few minutes to find it.


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## Paul Running (Apr 12, 2020)

GreasyB in YYC said:


> I was kind of hoping to incorporate a tube rectifier, because all my builds have used a bridge rectifier and I wanted to try something different.
> 
> What I'm interpreting from everyone's ideas is that the PT could work in almost anything, depending on how the B+ is managed.
> 
> However, the limitations to build variety result from the OT. Is that correct? Depending on speaker values and wiring configuration, could this OT be used in a Tweed Deluxe, for example?


Some people will mention that the OPTFM is the heart and soul of a tube amp and I agree. I believe that when considering a design choose your critical components first, the OPTFM, output tubes and PTFM and design around those components. Knowing the characteristics of these components should provide a clear picture in your mind of how it will evolve.










Your PTFM HV secondary is center-tapped for 185VAC full-wave rectification so, the B+ would be low at 260VP @ 900mA; for higher voltages, a bridge has to be used.
The PTFM does not provide a 5VAC secondary to run 5-series type rectifiers however, you could use 6.3VAC indirect-heated cathode tube-rectifiers or source 5V to heat 5-series type, rectifier tubes. If you want extra headroom, use semi-con rectifiers for the plate supply (red-red), they will produce higher voltage with very little sag. The DCR will be low on the HV winding...450mA for a 50W amp, it will operate very tight.


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## tomee2 (Feb 27, 2017)

Use a pair of 5ar4s as a bridge?
There was a a high end stereo amp maker years ago that used a silicon diode full bridge rectifier but added a 5ar4 right after it to provide slow turn on and some voltage drop. Probably a marketing gimmick as much as anything actually useful.

Edit..no 5V winding...so no 5ar4s!


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## Lincoln (Jun 2, 2008)

tomee2 said:


> I think rob robinette has something to say about converting a twin reverb to use 6v6s so maybe 40 watts instead of 80. Or about using only 2 tubes instead of 4 but changing the speaker tap.
> Similar problem here but with 4 EL34s instead of 6L6s.


I was reading that part last night. He says you can put four 6V6's in a Twin Reverb, but you have to double the impedance on the output. For example, a normally 4 ohm output with 6L6's becomes an 8 ohm output when you use 6V6's. That would make those transformers the OP is asking about usable as well.


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## tomee2 (Feb 27, 2017)

Lincoln said:


> I was reading that part last night. He says you can put four 6V6's in a Twin Reverb, but you have to double the impedance on the output. For example, a normally 4 ohm output with 6L6's becomes an 8 ohm output when you use 6V6's. That would make those transformers the OP is asking about usable as well.


Yes, that's what understand as well. And then if you pull 2 6v6s, you go up again and the 4 ohm out becomes 16. 
Severe overkill amp at that point, 450mA power transformer delivering maybe 120mA?
But it's a way to tame a Twin Reverb.. To build from scratch around this idea might be foolish, not sure.
But I love Rob's website. I've not seen anything there that's not well explained and he puts safety first.


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## Paul Running (Apr 12, 2020)

The impedance ratio for the TR is 2K1 ohms plate to plate : 4 ohms load is 525:1 and 8 ohms is 263:1.
4 X 6V6s would be 3K5 ohms plate to plate impedance to an 8 ohm load which is a ratio of 438:1. You may wish to decrease the NFB for the 6V6s...the effect should lower the plate impedance for a closer match to the speaker(s).









TR OPTFM top, DR OPTFM below. (note that it is for 2 6V6s...it caught me at first).


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## nonreverb (Sep 19, 2006)

Paul Running said:


> The impedance ratio for the TR is 2K1 ohms plate to plate : 4 ohms load is 525:1 and 8 ohms is 263:1
> The impedance ratio for the DR is 7K ohms plate to plate : 8 ohms load which is 875:1. Operating those 6V6s at 525:1 ratio may result in lower performance...remember that with high-powered devices, maximum power transfer occurs peak, when the impedance is matched from stage to stage, in this case the plates of the output tubes to the reflected-impedance of the load which for our applications is a speaker or speakers. The TFM is an interface that can smoothly extend a friendly hand-shake between the output tubes and the speaker. When ultimately tuned a HQ OPTFM can run greater than 95% efficiency.
> It is very dependant on the impedance ratio...remember that and you've got it half in the bag.
> View attachment 347943
> ...


Indeed Paul...also note the plate voltage is significantly higher in a Twin. You'd be running standard 6V6's hard. Perhaps the best route would be JJ6V6's as an option as they spec higher than the regular offerings.


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## Paul Running (Apr 12, 2020)

nonreverb said:


> Indeed Paul...also note the plate voltage is significantly higher in a Twin. You'd be running standard 6V6's hard. Perhaps the best route would be JJ6V6's as an option as they spec higher than the regular offerings.


Yes I agree...I wonder what their success rate is for matching quads...are the production lines well controlled for tolerances? I have not investigated this yet.


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## tomee2 (Feb 27, 2017)

I think robinette recommends those as well, good to 500V? 
I mean who doesn't want a 20watt head that weighs 50 pounds and has transformers that don't even get warm. Haha!


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## nonreverb (Sep 19, 2006)

Paul Running said:


> Yes I agree...I wonder what their success rate is for matching quads...are the production lines well controlled for tolerances? I have not investigated this yet.


Well...like all new production tubes, there's always the possibility of a dud or two. That said, they're an improvement over typical Russian or Chinese product...at least where op. spec. is concerned.


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## jb welder (Sep 14, 2010)

You probably know this already, but higher power transformers are worth substantially more than lower power. So it's kind of a waste to put them in a low power build, and you get penalized with the extra weight.
You mentioned a bass amp, so a 60 to 100W bass head would be a good option. Otherwise, I think you could swap for lower power iron and probably come out with some money to boot.


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## GreasyB in YYC (Jan 6, 2020)

Thanks for everyone’s contributions. I put the question out there in case there was a simple, straightforward solution that I was missing. However, the square peg, round hole analogy seems to be the consensus. Thanks again and hope y’all are keeping healthy and sane!


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## Paul Running (Apr 12, 2020)

GreasyB in YYC said:


> Thanks for everyone’s contributions. I put the question out there in case there was a simple, straightforward solution that I was missing. However, the square peg, round hole analogy seems to be the consensus. Thanks again and hope y’all are keeping healthy and sane!


Yes, it is helpful to make informed decisions...make a decision that is good for you...the best to you.


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