Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxbox on January 04, 2017, 03:50:19 pm
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Hi, its been a while since I've posted here.
The amp has had the 5881s replaced by EL34s by the owner, after I had biased it 6 years ago for new 5881s.
I believe that EL34s cannot be used in the 5881 circuit without some mods? Is that correct?
The amp came in with the report that it was dead, but its been running on 75% power into a dummy load fine and there don't seem to be any dry joints, so they've probably just operated one of the two switches on the back that can kill the sound without knowing it.
Main thing is the power valves. Its not a straight swap between the 5881 and EL34 - what issues would there be if the amp ran on the EL34s? :w2:
Cheers, VB
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5881 has a heater consumption of 0.9A, EL34 is at 1.5A
this can be a problem if the PT isn't adequate
this is the pinout of the EL34 (http://tdsl.duncanamps.com/basing/8ep.gif)
this is the pinout of the 5881 (http://tdsl.duncanamps.com/basing/7ac.gif)
To say if there are problems you must give a look to the wiring on the socket
Franco
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Grid stoppers are normally larger as well as what K mentioned. 1K5 i believe. I use 5 watt ceramic blocks usually.
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I believe that EL34s cannot be used in the 5881 circuit without some mods? Is that correct?
Maybe. EL34s require pin 1 to be connected to pin 8, but not so for the 5881. Since this is a Marshall amp chances are high that pin 1 and 8 are already strapped together.
EL34s require less negative bias voltage on pin 5 than a 5881, but if the bias pot has enough swing you don't have to do anything except readjust the bias for the EL34s.
Screen current is usually higher for the EL34 so that usually requires a larger screen resistor.
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Ha! Eureka!! :icon_biggrin:
The owner of this amp actually has two of these, with the gold (brass?) plated chassis. One was built with 5881s, however this one is with EL34s..... doh. And properly built for EL34s.
It doesnt help when Marshall publish a document with the filename "5881 to EL34 conversion", when the actual content is to convert from EL34s to 5881s........ :BangHead:
Cheers, VB.