Old guy returning and need some advice on a 1624T Supro amp build

I have been away from the tube amp world for a few years. I am going to build a 1624T Supro syle amp with 12AX7 pres and EL84 power tubes. The amps were built using 6973 which I can get NOS but pretty costly, around $100 each. Am I making a mistake by using EL84s?
I have in mind to use a vintage greenback 12" speaker which is 8 ohms. I also have a Webber alnico 12" I could try also.

Two questions.
Is there any good reason to use the typical 5Y3 or can I just use diodes?

Do you have any recomendations for the PT and OT?

I will keep one hand in my pocket...lol

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No reason not to use EL84 that I can think of, and the 5Y3 would provide some sag which is probably good for your tone, other things? I'm a bit outside of my area of expertise here.

Welcome to the forum! :D

I expect others will weigh in with good parts sources, but check out Antique Electronic Supply and Handmade Audio.
Thanks kevin
I think my accounts may still be active with several suppliers. Handmade Audio is new to me. Magnetic Components Inc. was the original supplier for the transformers and they are still in business. I just would like to know what other people have used and what their experence has been.


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My brother is a guitarist and together we built a 1624T clone a few years back using whatever was to hand. Namely 5CZ5s instead of 6973s and 6EU7s in place of 12AX7s, a power transformer with 5V and 6.3V windings and an HT winding that was too hot. The 5/6CZ5 is near electronically identical to the 6973, much as the 6EU7 is to the 12AX7 (beware the different pinouts). The 1624T clone is in regular use as a gigging amp.

We used the 5Y3 but also considered solid state rectifiers with hefty power resistors in series to ensure sag under load. We used a fat power zener diode on the HT centre-tap to drop the HT. We took care to keep plate voltages below the maxima specified, especially building down here in Oz where the wall socket delivers 250VAC to the primary of a power transformer designed for 110/230VAC.

The max plate voltage for the 6CZ5/6973 is about 350V for a push-pull pair, compared to about 300V for the EL84. The 6CZ5/6973 has less gain than the EL84 - the EL84 is easier to drive but may well give less clean headroom. Not that that may matter ....

The 1624T input uses relatively high plate resistors (220K or more as I recall) on the 6UE7/12AX7 and has quite a bit of clean headroom. Not to say that the amp doesn't go filthy when pushed - it does, very progressively and with good control. Works well with pedals, which start to sound like what was promised on the tin.

For the OPT we used the reprehensible AUD$27 Altronics M1120 (here: https://www.altronics.com.au/p/m1120-20w-100v-line-pa-ei-core-transformer/) but could probably have used the AUD$19 M1115 (here: https://www.altronics.com.au/p/m1115-8-ohm-to-1.25w-15w-pa-speaker-transformer/) if we had only an 8 Ohm speaker.

Both the Greenback and the Webber are good choices, based on experience, with maybe more bass from the Greenback. If you want the ducks guts in 12" for working with pedals, try these: https://www.lorantz.com.au/loudspea...rs/12-inch-guitar/ac304p75-mi-12-inch-guitar/
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Here are my brother's comments, plus some pix of the build:

The old 5Y3 rectifier presents about ~ 350 Ohms resistance, according to Merlin Blencowe. This introduces a lot of sag in a push pull amp, and it's audible when you run the amp reasonably hard. It's part of the sound. Silicon diodes don't exhibit series resistance, but a similar effect might be achieved by adding a suitably high-powered 350 Ohm resistor in series between the recto circuit and the first filter. The absence of resistance in silicon also means higher voltages, and that could present problems of its own (see below re transformers).

My experience with Celestion greenbacks and Webers AlNiCo speakers is that the Celestion will be louder and deliver much more bass, and the AlNiCo Weber will be much more detailed at the top end. I found my home made Supro 1624 to be pretty bright, so I thought it benefited from the Celestion's roll-off above 6 kHz. However, I also thought it sounded good with a Weber 12A125. The Weber shifted the emphasis to the upper mids and made the amp kind of snarl. The Celestion made the amp sound bigger and smoother. Both sat well in the mix of a three piece rock / blues band. Both are good, but they're quite different.

I speculate that the EL84s may give less clean head room, being more sensitive than 6973s. My experience of the 6973 is limited to my Supro clone, and I find the amp to have a lot of headroom. It's not necessarily very loud, but I can run the volumes well above halfway before there's any appreciable overdrive, and even if driven flat out it's not as dirty as a Fender 5E3 Deluxe (Fender amp from the 1950s with similar power into a 12" speaker). However, the preamp in the 1624 has much larger anode load resistors than a typical Fender or Marshall, giving the Supro preamp a flatter load line. This would give it more head room, too. In practical terms, it's loud enough to gig and it sounds very good with overdrive pedals, and likes fuzz a lot.

My Supro uses an M 1120 transformer from Altronics as the output transformer. I used a power transformer from a kit for a tweed Deluxe because it's what I had lying around. It is a meaty PT, and I suspect it was designed to be suitable for either the tweed Deluxe or the Deluxe Reverb. All of this meant it was way too much PT for the Supro. The voltages were far too high throughout, which made it sound odd and completely neutered the tremolo. Lovely blue glow in the valves. Plinking sounds as they warm up. Not good.

I ended up putting a huge zener diode on the centre tap of high voltage secondary to tame the thing. This did the trick for the voltages, and also made the tremolo rock. It's now my favourite built-in trem. The Fender-style trems all seem to introduce the oscillation as far down the signal path as they can. The Supro does it right up front, on the cathode of the preamp itself, and as a consequence it bloody WORKS.


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Thank you so much for taking the time to describe your build.

I played through an original mid-sixties Supro at a shop in Miami, Florida. I liked the sound. I also had a couple of Zinky Blue Velvet amps from Bruce Zinky. I assume Bruce owns the Supro brand now.

This amp-build idea got started because of a three-string guitar project I had just finished. One was a typical cigar box, and one was a solid body. I am 78 and getting a bit too old to be standing on stage playing guitar...lol

The new amp will get used at home to record with and perhaps a few busking trips in the small town I live in.

The amp in this photo is a sort of Delux Reverb with two speakers that can be switched three ways.



Some other craziness I have built...lol This amp was an experiment trying to combine a Bassman and a Peavey 5150. I have never got it to work properly. Both play well but I could never figure out how to switch them on the fly.


Lewis and me playing in Los Angles, California.


I don't know a lot about electronics. I also have been away from all this for three or four years and I am having to relearn stuff...lol Don't ever get old...lol


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here's a build, where he says that the OT is nearly identical to the Tweed Deluxe 5E3 with
an 8 - 10K primary impedance:

The Deluxe Reverb has a 6.6K primary.
Many of the Fender transformers are rather small, and the 18W Marshall at Mojotone might be
a good choice.

The Tweed Deluxe 5E3 is a very popular amp so there are cheap transformers out there. I just bought
this one before I noticed that it has a 6.6K primary which is not exactly correct but will work:

Here's a more expensive and correct 8K primary one, note that it is 1.5 lbs:

Here's the Marshall from Mojotone 2.5 lbs:

Also consider if you just need an 8 ohm out or say 4 and 8.
The 6973 is a Beam Power tube with a pretty high plate voltage rating, it is probably
similar to the 6V6, check the specs. The EL84 is a Power Pentode.
But we know that 6V6GT s work with 6 to 8K OTs.
If you look at the last graph in this spec sheet you can get
over 20W from 6973 with higher impedance OTs. But it has been noted that if you
run the screen higher that curve might change significantly favoring lower impedance OTs:

JJ 6V6GT s have been noted to work well in guitar amps with high plate voltage.
If you want to get creative and use cheap tubes NOS 5V6 s are out there with lower plate voltage.
Roger Modjeski owner of RM labs posted this about his RM-10 hifi amp running 700V on the
plates of EL84 s, but about half that on the screens, on a forum back in 2001:
From: https://www.audioasylum.com/audio/tubes/messages/61139.html

And he gets 30W from a pair! I wouldn't worry about 350 to 400V with good tubes but do
take care of the screens.

I find that fascinating because so many just use the same old circuits.
His writing at that link is a bit confusing because he talks about both EL34s running with 800V on
the plates and producing 100W for a single pair! And the EL84s in his amp running with 700V on
the plates for 30W from a single pair.

He writes this "How many of you know of the published 100 Watt/pair design for El-34's run at 800V
plate, 400V screen? That came out with the tube in the 60's." About and article in the 1960s does
anyone know what article that was and where to find it?
Hi guys,

I was able to find a Hammond 272FX PT 600VCT and a 1760 H OT for super cheap. I also found this schematic. I just got the PT which is new in the box and I will test the output votage.

The mains where I live run at 124VAC.

The amp will only be used at home to record with. I also found a used Supro DT 15 made by Eminence.

I will pre drill and cut the holes for the tube scockets with my CNC mill and then take it to a shop to use their sheet metal break to bend it up.

After I get this build done I will find a NOS set of 6973 tubes and mod this amp to run them.

Also, this is one of the original schematics.




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