Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on December 25, 2020, 11:39:45 am
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Anyone know if this amp is cathode biased? New from Marshall 20/5 watt version of a 1959 SPL 100 watt head. Three 12ax7's, two EL34s. Suppose to sound just like a 1959 SPL 100 watt head but way lower volume, no master, solid state rectifier and I assume cathode biased as the amp is just 20 watts. Anyone have any info on this amp.
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I don't think "SV20VH" is its right name. Maybe too many "V"s? Marshall models are already too complicated for me to figure out.
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https://www.marshallforum.com/threads/is-sv20-really-cathode-biased.106866/ (https://www.marshallforum.com/threads/is-sv20-really-cathode-biased.106866/)
Marshall Service & Sales Support:“Hello,This item is cathode baised and just needs the EL34s to be matched when replacing.KEVIN DRURYMARSHALL SALES AND SERVICE SUPPORT1649 BARCLAY BLVD. BUFFALO GROVE IL 60089
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Anyone know if this amp is cathode biased? New from Marshall 20/5 watt version of a 1959 SPL 100 watt head. Three 12ax7's, two EL34s. Suppose to sound just like a 1959 SPL 100 watt head but way lower volume, no master, solid state rectifier and I assume cathode biased as the amp is just 20 watts. Anyone have any info on this amp.
Yes its cathode biased. I have one. Great amp.
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High Voltage, Great! We're all waiting on a schematic & layout..... :icon_biggrin: Merry Christmas
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Well, yeah... :icon_biggrin:
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under 400V B+ - looks like sand-state buffering for the effects loop, at least they're using decent op-amps (TL072s).
me thinks about 375V to plates and wind screens back to around 300V, they hit close the 20W target with a +/-3.5KΩ a-a load. probably closer to 25W
http://marshallroadhouse.com/viewtopic.php?t=21968&start=30
looking at those nice high rez shots - looks like maybe 2 B+ supplies, bipolar 12V for op-amps, relays & possibly DC heaters for the preamp tubes.
PT connections
* red-red for EL34 plate supply - probably around 370-380V (100uF 400V + 2 x 47uF 400V caps in this PS).
* wht-wht for EL34 G2 and preamps - probably around 300-325V (47uF 350V cap in this PS).
* yel-yel for the 6.3VAC for the EL34.
* grn-grn for the +/-12V for the sand state & possibly the preamp heaters. (4700uF 25V caps in this PS).
* blu for the bias supply?
take all the above with a grain of salt, not really able to trace much as most of the traces seem to be on the bottom layer of the board. don't see any evidence of a VVR for the screens or any of the other power bits.
--pete
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I don't think "SV20VH" is its right name. Maybe too many "V"s? Marshall models are already too complicated for me to figure out.
It's Model SV20 H, the H is the head version
Schematic will leak out soon, was thinking of converting one of my Plexi 6V6s to this amp, shouldn't be too hard. Getting the screen voltage down, might be an issue I guess use a resistor to reduce the voltage as Pete mention. If that blue wire is a bias tap, wonder what it's used for since this amp is cathode biased, I see a 150 ohm R. and cap. Great reviews, I'd skip the effects loop and relays if any.
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under 400V B+ - looks like sand-state buffering for the effects loop, at least they're using decent op-amps (TL072s).
me thinks about 375V to plates and wind screens back to around 300V, they hit close the 20W target with a +/-3.5KΩ a-a load. probably closer to 25W
I gather it's cathode biased, 150R. Preamp voltages about the same as a real plexi. EL34 anode and screens around 280V in 20W mode and 170V in 5W mode. So a sort of fixed value switchable power scaling idea.
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Here are the voltages I traced when I first got the amp. Bear in mind i'd only been messing with amps for around a month or so when I did this. The left one is in 20w mode and the right one is 5W mode.
Apart from that I also measured the output transformer and calculated the equivalent of 5.3K primary for the 4 & 16ohm taps and curiously 4.7K for the 8ohm tap (these were the measured impedance ratios multiplied by 4, 8 & 16). I'm still learning about power sections but I suspect they leverage the higher primary than the normal 3.4k/3.7k to squish down the loadline and make it more inefficient? Feel free to correct me if im wrong there.
I'd really be interested in further analysis from you smart people if these measurements help. Its an amazing amp, and you need to be in proper cork-sniffing zone to hear the differences from the real plexi. They really did a great job.
(http://)
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Thanks for posting the voltages, l’m in the process to changing one of my 6v6 Plexi’s as close to this new Marshall 20 watt you have. I’m a little confused as you say the a/c on the red wires was 435, l assume you mean both, DL posted pic’s above from another forum and took a guess of what’s what, he thought the white PT wires when to the screens, but you say your white wires had 435a/c to rectifier then to where? Am I correct? Also you have the bright side first stage K preamp at 820 rather the common Plexi 2k7...?
I was thinking that the lower wattage from this amp has a lot to do with lower plate voltage and much lower screen voltage? Are screens to the EL34s much lower than the plate. What are the EL34 screens and plate voltages.
Al
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Yeah as I mentioned this was way back when i didnt really know what i was doing, and thats not saying I particularly know what im doing now :)
I'm fairly confident the pin readings were correct, less so about the PT wires. Also I remember trying to trace the PT wires through was really tough. I wonder now whether there was some VVR going on even though someone above said they couldnt see any.
The plates were ~255VDC and the screen B+ node was 250 and 230 at the G2 pin (V4 & V5 on my chart).
If you super duper really really want me to open it up again to take some readings or look at something in particular Im happy to do so so long as I dont need to lift the board again (nightmare!).
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If you super duper really really want me to open it up again to take some readings or look at something in particular Im happy to do so...
Na, l’d never ask that, was more interested in the plate and screen voltage which you gave me, thanks.
Did you buy it new, if so no schematic..?