Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on August 25, 2024, 01:57:15 am
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He guys,
Can this be done using a typical Marshall PI and power amp and still retain the Marshall goodness
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Ciao TIMBO
I see an Lpad on your way
https://www.tubetown.net/ttstore/en/l-pad-at-62-mono-100w-8-ohm.html (https://www.tubetown.net/ttstore/en/l-pad-at-62-mono-100w-8-ohm.html)
Franco
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10 W power? or "sounds" 10 watts loud?
if it's choked to 10W power I don't believe any goodness will be let out :icon_biggrin:
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I thought the "Marshall Goodness" was generated in the pre-amp. 10W is like 1/3 of the EL34 power dissipation, kind of cold.
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PP EL34 Max Output 10W
Can this be done using a typical Marshall PI and power amp and still retain the Marshall goodness
DISCLAIMER: I haven't tried this, but...
I think it can be done. Lower the supply voltage, so that your plate & screen voltage is down around 250v (or possibly less). Page 6 of this EL34 data sheet (https://frank.pocnet.net/sheets/010/e/EL34.pdf) has curves for 250v on the screen.
I arbitrarily tried loadlines from 250v plate to 200mA, and 100mA. The latter seemed to land on "10w RMS" and is 200v / 100mA = 2000 Ω (Blue line). But this is the load to one side of the power section, and since Class A looked feasible here, that means 2kΩ x 2 = 4kΩ plate-to-plate (Red line) which is a common OT size.
Idle Bias looks to be about 17.5v.
The load line hits 0v well below the knee of the curves, so screen current might be high. It would be wise to scope the volts across a screen resistor while driving the power section to distortion to understand how/when screen current spikes up. Then you can decide whether a larger-value screen resistor is needed or not.
If it were my project, I'd try to build some sort of mock-up and hear whether the power section (with power supply and phase inverter) sounded good to me, then work on adding a preamp.
Consider that a typical Marshall 50w bias is about double what your output tubes will have... This means a standard Marshall preamp will tend to distort this power section at about half the drive-signal-level as found in a normal Marshall. You may find a need to work on pulling back preamp gain and/or mitigating output tube grid-blocking, etc.
(https://i.imgur.com/QKahDK2.png)
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Thanks guys,
I did a project many moons age where the Marshall style preamp was overdriving the 6GW8 power section and part of the solution was to split the plate resistor of the PI
Worked a treat without sounding crap.
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Hey guys,
Had a bit of a think about the power supply needed.
It probably wont be an off the shelf PT
First the heater supply would be quite large if a 6.3v were to power 2xEL34 and 4x 12ax7
Then to get a split HT that would supply Approx. 350v for the preamp and 250v for the power amp.
(https://i.imgur.com/9yRPHS2.jpg)
The local electronics store have an array of low voltage transformers that serve well for back to back power supplies.
All up $60
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Ciao my friend
I'll put attention on feeding 9v winding with 12v, it won't be a good idea, the second transformer will be prone to become HOT
You can do a 12v into a 12v
or a 9v into a 12v
BUT avoid to do a 12v into a 9v
Franco
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Thanks mate.
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... First the heater supply would be quite large if a 6.3v were to power 2xEL34 and 4x 12ax7
Then to get a split HT that would supply Approx. 350v for the preamp and 250v for the power amp. ...
If I were designing this amp, I wouldn't bother with a separate, higher voltage for the preamp.
The power tubes will have about half as much bias voltage as is typical for EL34, so the phase inverter only needs to deliver about half as much output signal. Therefore, a lower-voltage preamp is plenty fine.
Had a bit of a think about the power supply needed.
It probably wont be an off the shelf PT
I would spend a little on the iron.
Edcor 300v 120mA, 6.3v 4A PT (https://edcorusa.com/products/xpwr196-300v150-0-150-120ma-6-3v-4a-5v-2a)
Edcor 5H 200mA Choke (https://edcorusa.com/products/cxc100-5h-200ma-5h-200ma-choke)
The intent is a choke-input power supply, which should work well with a Class A amp that has a steady current-draw. This delivers 0.9 x VAC from the PT, so we assume about 270vdc minus a couple diode-drops.
Yes, this is expensive (about $180 USD), but "inexpensive" went out the window with the decision to get 10w from big tubes instead of typical tubes like 6AQ5/6BQ5/6V6/6K6/etc.
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Try this one with a metal shield:
https://www.antekinc.com/as-1t200-100va-200v-transformer/
https://www.antekinc.com/ca-100-steel-cover/
--Pete
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Thanks guys.
@HBP
The valve amp market here has hit an all time low.
I have a bunch of vales that would accommodate 30w+ but it seems here no one wants these amps.
Even 20w are hard to sell
So 10w using PP EL34
:dontknow:
I have a couple of PT's that could do the job but have then earmarked for other builds
I like the low voltage PTs for experimenting
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The valve amp market here has hit an all time low.
welcome to the 21st century :icon_biggrin:
everyone here is full-on digital/SS, the 'ol guys with tube amps are, well old, so they noodle at home on a 5 pound SS amp