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Schematic  Perhaps Gordon or Trev could answer this question. The original amps used a 6973 valve/tube. I want to use EL84s. The 6973 max plate voltage 410V and 330V on the grid. What would I need to change on this schematic to use EL84? 300V plate and grid max. Thanks, Billy
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I'm away sailing at the moment and I'd need to research the valves, which isn't easy from here. Assuming the valves are generally similar, it'll likely work OK as valves are self biassing. They tend to be more forgiving than transistors.
If you don't have an answer by the time I get home again, I'll look into it.
Jazz relative beginner, starting at a much older age than was helpful.
Kawai MP6, Korg M50, Ui24R, Saffire Pro 40.
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Thanks, Gordon
I am not in a rush. I hope you are enjoying the sailing.
We can kick this idea around when you get back. I may bite the bullet and buy a couple of 6973 valves. Electro Harmonix is making them again.
I may build the amp just like the schematic. It probably will not sound like to original in any case. The company that built the original transformers is still in business and I am researching that issue now.
The EL84 valves will have less headroom and break up quicker. That may be a good thing.
A quote from a friend of mine who has built a Supro.
"EL84s
I speculate that the EL84s may give less clean headroom, 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 headroom, too. In practical terms, it's loud enough to gig and it sounds very good with overdrive pedals, and likes fuzz a lot."
Both valves use the same socket but have very different pinouts. As I intend to build this on a turret board I could rewire and try both valves.
I have never been very skilled at electronics and everything I know has come from repairing amps and building them. I have never been to school or had any training.
If I use the original PT the voltage will be too high for the EL84 valves. I ASSUME I could use a zener diode to reduce the voltage. That is a bit over my head to design.
Fun stuff at any rate.
I am not sure where Trev is at the moment. He told me he was going on a long trip to the western coast in his new caravan. I hope he will post some photos.
Cheers,
Billy
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Hi Gordon, I found this schematic that Steve Lucky drew in Viso. He does really nice drawings. This was to make things work with EL84 valves. I will want to give this a try, but I decided to build the 6973 version first. I hope it will sound close to the original vintage Supro amps. After the build is finished, I may see what you think about modifications that may be needed to change the tone. No two amps sound exactly the same, and although Magnetic Components is still in buisness making PT and OT, I assume they are not exactly the same as 1965.   By the way, I had a conversation with Trev today, and he is in the middle of a real expedition in the western part of the country. I hope to see some photos soon. Some kangaroos playing guitar to BIAB tracks...lol Cheers, Billy
New location, new environment, new music coming soon
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By the way, I had a conversation with Trev today, and he is in the middle of a real expedition in the western part of the country. I hope to see some photos soon. Some kangaroos playing guitar to BIAB tracks. With a Tamworthnose amp in their pouch!
I am using the new 1040XTRAEZ form this year. It has just 2 lines.
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Once upon a time.... (true story)... I was a poor man who played guitar (is that redundant?) and badly needed a good amplifier. However, being poor is a major problem in that regard. I had cobbled together a few things to play with but nothing really worth the time. I wanted a Marshall head and cab but.... too costly. So, since I had some electronics experience, I figured I could gather the components and build one. I obtained a schematic for the 50w head and bought the transformers and tubes.... picked up the resistors and caps and started my build.
I ended up getting a good deal on a used amp from a kid down the street who needed cash for some legal issues he had gotten himself into, so the project was shelved for a time. Then I had a bit of time and money some years later and decided to pull out the project, blow off the dust and complete it.
With it proudly assembled .... on an open chassis with dangerous things available to the curious fingers.... I took it to the gig I had at the time.... house band in a local night spot. I set it on the 4x12 cabinet, plugged it in, and let it rip. Man that thing sounded really, really good..... for about 30 seconds or so.... them it started getting really distorted and the sounds started getting wonky and I noticed the output tubes looked like light bulbs.... oh my.... I don't think they're supposed to be glowing that brightly.... then pop.... the circuit breaker on the amp kicked off. I was smart enough to include an emergency circuit breaker in the design. Seems the bias was off by a considerable margin on the grids and it just went into runaway once I started playing. Since I had my Mesa at this time, I decided not to pursue the goal of building this amp. I unplugged it, set it on the floor to cool off, and put the Mesa back on the cabinet.
I still have all the parts and occasionally I think about reworking it and figuring out the correct bias, but then I think.... nahhhhh.... maybe next week.
You can find my music at: www.herbhartley.comAdd nothing that adds nothing to the music. You can make excuses or you can make progress but not both. The magic you are looking for is in the work you are avoiding.
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Well, Herb, here is waht you need. Takes all the work out of the bias issue. https://www.eurotubes.com/store/pc/bias%20probes.htmI have a Deluxe Reverb style amp I built some years ago. I made some mistakes with the Tolex cover and the grill cloth. I decided to fix it today. After putting the amp back together it has a hum. I knocked something around I guess. I never really was 100% happy with the sound so I guess this the time to rework the amp. Most of these tube amps are only really useful for playing on stage. Not so mant are good recording amps. Billy
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Man that thing sounded really, really good..... for about 30 seconds or so.... them it started getting really distorted and the sounds started getting wonky and I noticed the output tubes looked like light bulbs.... oh my.... I don't think they're supposed to be glowing that brightly.... This is one of the factors why I didn't try to make too much comment from my boat. I'd want to do the research. Most amplifiers of this kind operate in what's called "class-B", in which mode the amplifier dissipates a similar amount of heat to the output power it's delivering and that dissipation is what makes the valves (or transistors) get hot. It's likely where your glow came from, though there are other possibilities. BTW, Billy, I doubt that any kind of zener would be suitable to drop the voltage for your EL84s ... If you want to use those, you should try to find a suitable power transformer with appropriate winding voltages.
Jazz relative beginner, starting at a much older age than was helpful.
Kawai MP6, Korg M50, Ui24R, Saffire Pro 40.
AVL:MXE Linux; Windows 11; Win8.1: Scarletts
BIAB2022 UltraPAK, Reaper, a bunch of stuff.
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No two amps sound exactly the same That's certainly true of valve amps as they tend to rely somewhat on the valves themselves to keep things under control. With semiconductors there usually (invariably, I suspect) also some overall negative feedback that makes the behaviour more tightly controlled. In general principle, that's a good thing, but with valves there's kind of "essence of life" from the valves themselves.
Jazz relative beginner, starting at a much older age than was helpful.
Kawai MP6, Korg M50, Ui24R, Saffire Pro 40.
AVL:MXE Linux; Windows 11; Win8.1: Scarletts
BIAB2022 UltraPAK, Reaper, a bunch of stuff.
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Here is Steves schematic using 6973 valves using the same transformers as the schematic for the EL84. Notice that the B+ voltage is 347V at the plate of the 6973 and 330V at the plate of the EL84/6BQ5. Steve sometimes is not worried to push the valves a bit. 300V is the max rated voltage for a EL84. I have run them a little hotter in the past without issue. As I remember the plate voltage in one of Bruce Zinky amps runs over 300V. There is a resistor change in the power supply that is not on the schematic.   Billy
Last edited by Planobilly; 07/23/23 08:21 AM.
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Hi Gordon, I found this schematic that Steve Lucky drew in Viso. He does really nice drawings. This was to make things work with EL84 valves. I will want to give this a try, but I decided to build the 6973 version first. I hope it will sound close to the original vintage Supro amps. After the build is finished, I may see what you think about modifications that may be needed to change the tone. No two amps sound exactly the same, and although Magnetic Components is still in buisness making PT and OT, I assume they are not exactly the same as 1965. By the way, I had a conversation with Trev today, and he is in the middle of a real expedition in the western part of the country. I hope to see some photos soon. Some kangaroos playing guitar to BIAB tracks...lol Cheers, Billy I put the two schematics closer together.
Last edited by Planobilly; 07/23/23 08:19 AM.
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Photo of the amp Steve built.  Billy
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My very first amp was a little Supro. It had one 12 inch speaker, spring reverb and tremolo. I remember it was red. That's all I remember about it though. I played that for about 2 years until I got a Fender Twin as a birthday present. Not long after that I added a 4x12 cabinet. That was in the mid 60s so it was still tubes. What a rig I had for a 15 year old kid!! With that amp sitting on that cab I looked like I knew what I was doing. Playing an ugly as sin red Fender Jazzmaster with that awful tortoise shell pickguard. The all white Jazzie I brought to Herbstock had that pickguard until I had it wrapped in vinyl to have white on white.
Eagerly looking forward to hearing that amp you are building.
BEFORE it catches fire.
I am using the new 1040XTRAEZ form this year. It has just 2 lines.
1. How much did you make in 2023?
2. Send it to us.
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LOL, Eddie, I have had one or two smoke, but I have not had to use the fire extinguisher I keep by the workbench.
I am reworking a Deluxe Reverb I built a few years back at the moment.
I am trying to get in contact with a custom transformer guy I know to order an output transformer from him. I have not done anything with amps for a while. The pandemic has screwed up the supply chain. Companies have gone under.
I found a metalworking shop here in Sebring that will make chassis for me at very reasonable prices.
I have wanted a Supro for a while. I just could not choke down the price for a mid-sixties amp.
Billy
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I'd love to help you more, Billy, but valve / tube technology largely pre-dates my electronics experience. I really started with IC's, semiconductors and microprocessors. The only vacuum tube tech stuff was what I taught myself.
Your amp looks great though, I'm sure it will be fantastic.
Trev
BIAB & RB2024 Win.(Audiophile), Sonar Platinum, Cakewalk by Bandlab, Izotope Prod.Bundle, Roland RD-1000, Synthogy Ivory, Kontakt, Focusrite 18i20, KetronSD2, NS40M Monitors, Pioneer Active Monitors, AKG K271 Studio H'phones
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Hi Trev, Most of my questions are pretty general. Signal injection testing, capacitor value relation to frequency, that sort of stuff. I posted the photo in another thread. Here is the finished chassis.  I changed my mind and built a S-6424. After changing the value of the cathode bias resistor value, which caused the amp to run at 70%/24 ma, it sounds pretty good. Now I have to make and cover a cabinet. Also, make some sort of face plate. The 15" speaker arrived, but I don't like the sound as well as the vintage Pre Rola Celestion Greenback. I have four of these speakers. I am not sure I want to put a five or six-hundred-dollar speaker in this amp. I think I have one more music project, a rack-mounted tube Tremolo and Reverb. I think the time has come to get my hands out of all this 600-volt stuff. I may try to build some effects boxes, all low-voltage transistor stuff. Cheers, Billy
New location, new environment, new music coming soon
Seize the moo-ment
If you feel like you’ve herd all these cow puns before, you probably have deja-moo
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