Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PBJ on March 16, 2017, 04:29:26 pm
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I’m planning to build a low wattage plexi, a browner version, based on the ordinary 50 watt version but with lower B+ (and maybe add some scaling on top of that)? I guess for example that this low voltage power transformer in a 50 watt plexi would give me a 33 watt version http://www.mojotone.com/amp-parts/Transformers_1/British-45-Style-Low-Voltage-Power-Transformer (http://www.mojotone.com/amp-parts/Transformers_1/British-45-Style-Low-Voltage-Power-Transformer)? Maybe there are even some more lower voltage transformers on the market to choose from? Isn’t this approach more logical than using 6V6s or EL84s to keep the wattage down to some extent or am I missing something?
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The tubes operate at different ranges, the EL's both seem to be good around 250 or 300, but the 6V6 want's more like 285, so you'd have to adjust there as well to find similar operating characteristics I'd think. If you're going for lower wattage and want most other things to be equal, then it seems like the EL84 is similar to the El34 in characteristics. Obviously biasing is different, though so they'd need different bias resistance values etc. But to me, anyway, sounds like a decent idea. I'm still not super strong on what kinds of power tube combos work with what kinds of preamp topologies, (I've heard most are fine somewhere but then I also heard sometimes the impedance can mismatch and hit some tubes too hard, but I'm not sure how to validate that part yet, I'd love to understand it better)
~Phil
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I've an old Geloso G 227-A PA amp that uses a pair of EL34 in PP with 320v B+, at this low voltage the power is around 30-35W
See here http://digilander.libero.it/pasqua49/COLLEZIONE%20HI-FI/GELOSO%20G227A/Geloso%20G%20227%20A.htm (http://digilander.libero.it/pasqua49/COLLEZIONE%20HI-FI/GELOSO%20G227A/Geloso%20G%20227%20A.htm)
(http://i.imgur.com/HS0vnhO.jpg)
(http://i.imgur.com/NUJEpGo.jpg)
Franco
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Aim for 400V main B+, but use a 6.6KCT OT. This leaves plenty of B+ for the little stages, power output is 20W-25W.
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Could anyone please verify my "rough" B+ calculus attempt(s)?
Standard PT 360-0-360 VAC center tapped with a full-wave rectifier:
0.9 = load drop factor
1.3 = rule of thumb factor? (1.41 is max and will give the AC peak value)
360VAC*0.9*1.3 = 420 Volts
Low voltage PT 300-0-300 VAC center tapped with a full-wave rectifier:
300VAC*0.9*1.3 = 351 Volts
Besides, adding a tube rectifier would give a further 20-40 vdc drop. I think I want a tube rectified plexi in order to get more sag/compression (the additional voltage drop would be a bonus i guess). Besides, it would be possible to make it switchable to SS later on.
I guess my goal is to build some kind of a hot rodded JTM assuming that a plexi with some tweaks would take me there.
I will have a look at the 6.6KCT OT recommendation.
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Low voltage PT 300 VAC center tapped with a full-wave rectifier:
300VAC*0.9*1.3 = 351 Volts
I did a plexiish build with 34's cathode biased, plate volts came in about 330V. well received, my notes suck so I can't give you much more than that, notes show about 23W crunched, 17W clean. I believe my OT was 4.5k into 8ohms.
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SIM below of hammond 272DX at rated current load. should be close enough.
a design guide that's easy to follow:
http://www.hammondmfg.com/pdf/5c007.pdf (http://www.hammondmfg.com/pdf/5c007.pdf)
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Standard PT 360 VAC center tapped with a full-wave rectifier
Just for clarification...
A 360VAC center tapped is the equivalent of 180-0-180VAC. But you're talking about this as if you mean 360-0-360VAC. Be sure you have what you think you have.
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Standard PT 360 VAC center tapped with a full-wave rectifier
Just for clarification...
A 360VAC center tapped is the equivalent of 180-0-180VAC. But you're talking about this as if you mean 360-0-360VAC. Be sure you have what you think you have.
I´ve updated previous post so there can be no misunderstanding ;-). Standard plexi 50W PT according to datasheet says 360-0-360 VAC and the other lower voltage version 300-0-300 VAC.
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Aim for 400V main B+, but use a 6.6KCT OT. This leaves plenty of B+ for the little stages, power output is 20W-25W.
Could you please give me a hint of why you are recommending the use of a 6.6KCT impedance OT since this seems a bit off when compared to the standard 50 watt Plexi using 3.7KCT? Will this really be safe in all situations? If I got things right it seems like the TW-amps uses a 6.6KCT OT for both EL34 and 6V6, and if so, does this also make up for a good multi-tube OT impedance?
Standard: EL-34 Push-Pull, B+ 420 Volts -> 50 watts -> 3.7KCT OT?
Low wattage version: EL-34 Push-Pull, B+ 350 Volts -> 35 watts -> 6.6KCT OT?
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please give me a hint
you already gave yourself the hint :icon_biggrin:
B+ 420 Volts
B+ 350 Volts
look up the datasheet for a 34, look at the different plate volts then look at the Ra-a value
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low impedance OT with low B+ sucks more current. i think PRR recommended the 6.6K with low(er) B+ to keep current through OT and tubes manageable and since you'll be in class A territory.
take the single tube class A condition and double Ra-a: with 300V B+ you'd want a 7K Ra-a to make around 22W. 6.6K is a common part and is close enough.
--pete
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I start to be a bit confused about what bias-conditions the recommendations here are made for. Originally I had in mind fixed bias but with a lower plate voltage and in this case a 6.6KCT OT seems a bit odd although the TW amps seems to have this configuration. What I've read is that this type of mismatch alone will keep the power down but increase the risk of arcing which can destroy the OT. However, maybe a valid option would be to use a Trainwreck PT/OT in an otherwise standard Plexi 50 watt circuit? Or go for cathode biased EL-34s?
From datasheet:
EL-34 Push-Pull (fixed bias), B+ 400 Volts -> 54 watts -> 3.5k Ra-a
EL-34 Push-Pull (cathode bias), B+ 375 Volts -> 35 watts -> 3.5k Ra-a
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another factor, with Cathode bias you'll probably need a few more Vac to drive the 34's than with fixed bias. The sheet I looked at showed about -33 fixed, 40 self biased.
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i use hoffman 4k super-reverb/bassman OT at around 400V B+ with EL34. IMO, it's their sweet-spot.
datasheet with cathode bias with 450V B+ ra-a is 6.5K - makes 40W. 375V with 3.5K ra-a- makes 35W.
--pete
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:icon_biggrin: I know this is an old post but I find it relative in 2019 since Marshall is doing something to the new vintage amps to use EL34's and get them down around 20w ? They say , brand new stuff so I have no idea if they are using power scaling to achieve this.