has anyone used a reverb driver as an output transformer?

[Jewellworks]

i tweaked my PP/EF80 lunchbox amp, and ive got it sounding great. now my friends want one! so im thinking about simplifying the circuit and basically building an 5F2A (Tweed Princeton) but using a SE EF80 instead. it should only put out about 2, 2.5 Watts, so ive been looking at using a Fender Reverb Driver as an Output Transformer. ill have to use a 4 ohm speaker to get the load down to about 12k (anywhere from 10k to 15k looks good on the load line) plus, its much smaller and will fit inside the lunchbox. but how would it sound? does a reverb driver have the same frequency response as a "regular" OT?

another question: im using a separate filament transformer and PT, and both of these and the OT will be right next to each other. i know the OT and PT should be at 90 degrees of each other, but what about the Filament transformer? should it match the PT? or the OT? (im guessing the PT)

ill post pictures and a link to a video for the PP EF80 lunchbox as soon as i get around to it.

 

[corliss1]

A reverb driver *is* an output transformer

Take any traditional Fender amp - make a speaker cable to an RCA jack, or use clip leads or whatever - hook up your reverb send to a speaker - play some guitar - you'll get sound. Maybe not great sound at that point in the circuit, but still.

It certainly won't hurt to try it in your application.

 

[2L man]

Hammond publish very good transformer datasheets but Reverb Transformers do not seem have max bias current.

Look how impedance effect to low frequency attenuation where higher impedance use often make significant drop to lows. There 1kHz output is set as 0db reference.

 

[schmee]

I have not found rotating/offset PT vs OT etc to be a very big deal. On some amps in oddball chassis I've built they ended up right next to each other and not rotated. No issue.

 

[2L man]

EI-core power transformer EM stray field is strongest straight out of the long side so it is best to direct it away from where OT will be placed. Coil is around the - of the E and highest concentraion of EM strays direct <---E--->

Search for example 5E3 chassis picture and that is PT align which is wrong. Generally early 60s and later all big brands tube amps PTs were aligned right.

 

[SerpentRuss]

I have used an 7.5K Hammond reverb transformer for a single 6CL6 build. It did fine. I think they're good for 3 watts and IIRC that's right about where the 6CL6 tops out. I would think a 6K6 might be another candidate for that transformer. If you haven't purchased one yet however, the champ OT that Antique Radio Supply sells for $20 is probably a better purchase.

EDIT: Shame on me for responding just to the topic, not the body of your post. I would think using the champ OT with a 16 ohm speaker might get you there. I would also consider EDCOR, where you can order exactly what you need. I've found that they get things to me in about 1/3 to 1/2 the time they specify.

This one with a 16 ohm speaker:

GXSE5-5.5K - 5W, 5.5K Ohms single ended tube output transformer.

Designhe GXSE series was designed originally for guitar tube amplifiers. This makes it well suited for other type of single ended tube amplifier. These transformers are precision gapped to adjust for unbalanced DC current. Primary (Input)The input is designed for a 5.5K Ohms impedance source...

edcorusa.com

This one with an 8 ohm speaker:

XSE15-15K - 15W, 15K Ohms single ended tube output transformer.

DesignMathematically designed to produce exact plate and screen voltages. These transformers are precision gapped to adjust for unbalanced DC current. Primary (Input)The input is designed for a 15K Ohms impedance source. Screen/Grid TapThis unit has a 40% screen/grid tap. Secondary (Output)The...

edcorusa.com

 

[telemnemonics]

My understanding is that Fender originally just used the Champ OT for the reverb driver OT when they developed the spring reverb circuits.
Exact same OT.
My Kendrick Champ OT is bigger than my Twin reverb driver OT though, so later they may have gone to the smaller OT since not much output is needed to shake those springs.

 

[Paul-T]

Rob specifies the 1750A (25k/8 ohm) for the AU7 micro. So yes, given the EF80s seem to like around 10k, it should surely be fine with a 4 ohm speaker. It's very pricey now though, in Europe at least, so I'm not sure if it's worth using vs any 5W SE OT, although there are also genertic substitutes , liek this tube-town: https://www.tube-town.net/ttstore/en/fender-style-ot-3-5-w-reverb-125a20b.html
I'm certain I've seen this from US suppliers too.

 

[Jewellworks]

Rob specifies the 1750A (25k/8 ohm) for the AU7 micro. So yes, given the EF80s seem to like around 10k, it should surely be fine with a 4 ohm speaker. It's very pricey now though, in Europe at least, so I'm not sure if it's worth using vs any 5W SE OT, although there are also genertic substitutes , liek this tube-town: https://www.tube-town.net/ttstore/en/fender-style-ot-3-5-w-reverb-125a20b.html
I'm certain I've seen this from US suppliers too.

ah! i forgot about Robs micro with the AU7 using the 1750A. i was looking at that transformer first, then i found the 125A20B for about half the price. i also like the physical size, since im trying to fit it into a lunchbox.
i also have a SE 5k w 4,8,and 16 ohm outputs that i could double up and have a 8 (on the 4) and 16 (on the 8) for a 10k load. and i have that in hand and wont have to buy anything. i guess it all comes down to what my friends have for a speaker cabinet.
my bigger concern was how a transformer designed for a spring reverb tank sounds to one designed for a speaker. if its lacking low end and/or warmth.

@SerpentRuss - i will definitely look at Edcor

@2L man - im still not clear on how to rotate the filament transformer. -to MATCH the PT? i understand the OT needs to be at 90 degrees from the PT. what about the 6.3v transformer?

 

[printer2]

Why not use a real power tube? 6AK6?

 

[Jsnwhite619]

My understanding is that Fender originally just used the Champ OT for the reverb driver OT when they developed the spring reverb circuits.
Exact same OT.
My Kendrick Champ OT is bigger than my Twin reverb driver OT though, so later they may have gone to the smaller OT since not much output is needed to shake those springs.

I went through some OT questions on a 6G15 I built a while back. Had to check the build thread, but Hammond had their specs labeled wrong or something on the site. I used both - they sent the correct replacement - and there was some difference, but not a huge one.

From the 6g15 thread:

Follow-up from Hammond. The website did have it labeled wrong, and the 1750AX (7500k primary) is the intended reverb driver for the 6G15, NOT the 1750A (22k primary) which is the standard 12at7 reverb driver. The website has already been corrected, and they are sending me the correct one this week.

 

[telemnemonics]

I went through some OT questions on a 6G15 I built a while back. Had to check the build thread, but Hammond had their specs labeled wrong or something on the site. I used both - they sent the correct replacement - and there was some difference, but not a huge one.

From the 6g15 thread:

Follow-up from Hammond. The website did have it labeled wrong, and the 1750AX (7500k primary) is the intended reverb driver for the 6G15, NOT the 1750A (22k primary) which is the standard 12at7 reverb driver. The website has already been corrected, and they are sending me the correct one this week.

Interesting, and of course the 6g15 had or has the 6k6 rather than 12at7, while the champ OT would be way too big for the reverb OT but early on Leo may have just used what was on hand and then later changed specs once a R&D project got some sales behind it.
Also I forget where I read that and my memory may be faulty!

Further, when Fender reissued the 6g15 they gave it a 6v6 which would probably not match the same OT as a 6k6?
I bought the reissue when they first appeared in the early '90s.
Swapped in a 6k6.

 

[Paul-T]

ah! i forgot about Robs micro with the AU7 using the 1750A. i was looking at that transformer first, then i found the 125A20B for about half the price. i also like the physical size, since im trying to fit it into a lunchbox.

my bigger concern was how a transformer designed for a spring reverb tank sounds to one designed for a speaker. if its lacking low end and/or warmth.

There might well be a difference but it's not necessarily a problem. I don't know, why the 1750A is bigger than the 125A20b - i do actually have both, and was keeping the former for a Princeton reverb and the latter for a friend's Champ micro on the basis I want a clean sound for the reverb and a dirty sound for the Champ.

Physically smaller might mean less lower end and perhaps more compression etc... but the GA-5 and 5C1 Champ have tiny transformers that are definitely close to that size and it's an intrinsic part of the sound for many!

 

[2L man]

When distorted low volume is in mind using loadline which is not "optimized" can be good. Tubes "efficiency" is low and examples of voltages and impedances published datasheets often are optimized for high power/distortion ratio. However nowadays high output power sometimes is a problem...

I have done many tests using very low OT impedances and no tubes have failed yet but other tests using very high operative voltages I have lost quite many tubes.

This photo is my "5F2 6V6 Silicon testbed" which I have used for my often impulsive tests and now has a 230V/33V power transformer toroid as OT. Hot Glued and aligned like that because wires were barely long enough.

Toroid core does not have air cap so it must saturate already because of bias current but low volume output does sound quite clean. High drive distort a lot and volume stay very low as well. Attached loadline "green Swing range" produce only 1W and 2nd harmonic distortion is already 20% because output clips when anode current come Zero. Also that 1W THD 20% assume 0% OT distortion.

Even when loadline angle is very steep Anode current stop climbing when control grid drive voltage meet Zero line so tube dissipation stay reasonable even the peaks are about 30W. So output clip first on bottom and then from the top!

This test did accomplish its coal and proof my assumption that Power Toroid for SE amp is too drastic "distorter" in SE aml but also that 6V6 is very sturdy tube which can be used "outtabox"

So called "doorbell transformer" can be useable for low power SE amps. Old battery charger is another place where that kind transformers were used but nowadays they are switchers.

I have also test PP OT in SE amp and it work but core saturation distorts and drop lows but so does Champ OT...

There are two audio transformer standards (600 ohm and 100V) which have been used as OT for low power amps where increase distirtion has not been the idea. If they interest searching term "Flea HiFi 600 100V" should bring useful info but Toroid and Doorbell transformer as well when they are included to search function...

 

[Kev-wilson]

I'm using a reverb transformer in both of my RobRob Deluxe micro builds, I know little of the theory but they work ok and sound nice.

It's 3.5w & 8ohm & single ended.

 

[2L man]

ah! i forgot about Robs micro with the AU7 using the 1750A. i was looking at that transformer first, then i found the 125A20B for about half the price. i also like the physical size, since im trying to fit it into a lunchbox.
i also have a SE 5k w 4,8,and 16 ohm outputs that i could double up and have a 8 (on the 4) and 16 (on the 8) for a 10k load. and i have that in hand and wont have to buy anything. i guess it all comes down to what my friends have for a speaker cabinet.
my bigger concern was how a transformer designed for a spring reverb tank sounds to one designed for a speaker. if its lacking low end and/or warmth.

@SerpentRuss - i will definitely look at Edcor

@2L man - im still not clear on how to rotate the filament transformer. -to MATCH the PT? i understand the OT needs to be at 90 degrees from the PT. what about the 6.3v transformer?

Direct 6,3V transformer strongest EM Strays also away from the OT!

 

[dan40]

If you are concerned with noise entering the circuit because of the transformer alignment, you can use the "headphone trick" to find the perfect mounting position. Simply wire up the primary of your PT to 120vac and then connect a pair of headphones to the secondary wires of your OT. Then it's just a matter of rotating the OT until you find the quietest position.

 

[2L man]

If you are concerned with noise entering the circuit because of the transformer alignment, you can use the "headphone trick" to find the perfect mounting position. Simply wire up the primary of your PT to 120vac and then connect a pair of headphones to the secondary wires of your OT. Then it's just a matter of rotating the OT until you find the quietest position.

This is fine but the PT should be aligned first so that its magnet field towards the OT is weakest!

 

[Supertwang]

i tweaked my PP/EF80 lunchbox amp, and ive got it sounding great. now my friends want one! so im thinking about simplifying the circuit and basically building an 5F2A (Tweed Princeton) but using a SE EF80 instead. it should only put out about 2, 2.5 Watts, so ive been looking at using a Fender Reverb Driver as an Output Transformer. ill have to use a 4 ohm speaker to get the load down to about 12k (anywhere from 10k to 15k looks good on the load line) plus, its much smaller and will fit inside the lunchbox. but how would it sound? does a reverb driver have the same frequency response as a "regular" OT?

another question: im using a separate filament transformer and PT, and both of these and the OT will be right next to each other. i know the OT and PT should be at 90 degrees of each other, but what about the Filament transformer? should it match the PT? or the OT? (im guessing the PT)

ill post pictures and a link to a video for the PP EF80 lunchbox as soon as i get around to it.

Instead of building/buying piece meal,…I suggest repurposing the transformer sets from old tube radios, tube organs, police com radios, old tube tape decks, etc.