Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on February 01, 2016, 10:11:00 am
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I'm repairing an old German Echolette 120M amp and have changed its power
tubes from EL503 to EL34 since EL503 are very difficult to find.
http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/EL503/M120.jpg (http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/EL503/M120.jpg)
I cannot get the tubes biased even the negative voltage is -25V on the grid.
Actually this the highest value I can get without changing the bias setting circuit.
In my opinion this should be enough for very high current.
Plate voltage is 470V
The voltage on cathode over 1Ohm resistor is ~20mV i.e. the current is 20mA
/Leevi
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Ciao Risto
Also italian FBT and Geloso amps used those tubes
the EL503 at the moment is very rare and expensive if found
John Chambers on his pages published a detailed version of his conversion to EL36 tubes
the EL503 are problematic to be substituted because of their request in current & voltage
an EL503 need 1.05A till 1.2A (depending on brand) @ 6.3v for filament and only 265v B+ on a 2.4Kohm load for a 40W output
http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL503.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL503.pdf)
this low B+ voltage is a problem that Chambonino solved adding a small PT in series to the original PT (on Geloso PA amp) and adding a winding for the bias
if you read his conversion report you can find a lot of intresting news
http://www.chambonino.com/modify/mod1.html (http://www.chambonino.com/modify/mod1.html)
Schematics
http://www.chambonino.com/modify/mod1a.html (http://www.chambonino.com/modify/mod1a.html)
Following Chambonino a friend of mine (Giulio) modified an FBT 1200R (old italian guitar amp) and solved the same problem
http://www.chambonino.com/modify/giumod1.html (http://www.chambonino.com/modify/giumod1.html)
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The EL34 tubes requires a higher heater current, 1.5A and same for the B+ voltage
I suppose that on the original PT of your amp there isn't enough current for the filament of the EL34 tubes
On your amp have you changed the PT ?
And ... what about the OT that I think is around 2.4k ? (this last problem may be solved of course connecting the speaker on a different intake)
Franco
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do you have -60V at c41 & r110 node?
--pete
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Thank for your response and the links Franco
The EL34 tubes requires a higher heater current, 1.5A and same for the B+ voltage
I thought this might be an issue, but the PT looks very huge and powerful. So far no problems but let's see.
only 265v B+ on a 2.4Kohm load for a 40W output
Before the change B+ was 470V and in the schematic it seems to be even higher.
The 2.4kOhm load resistance will be optimal if you plug an 8 Ohm speaker to 5 Ohm output so you'll get ~3,4kOhm
/Leevi
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I was missing the B+ voltage on the schematic ..... 500v on the plates of the EL503 :huh:
this is the first time I've seen those tubes used at a so high voltage .....
If you have a so high voltage your conversion will be much more simple
Ciao
Franco
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do you have -60V at c41 & r110 node?
-57V
/Leevi
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do you have -60V at c41 & r110 node?
-57V
/Leevi
what do you measure at the wiper of the pot (node 5) with the pot at max and pot at minimum? <<< pull the power tubes to keep them from red-plating. measure unloaded B+ as well.
BTW, no offense intended, but did you strap pin8 to pin1 on the EL34 sockets?
--pete
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what do you measure at the wiper of the pot (node 5) with the pot at max and pot at minimum? <<< pull the power tubes to keep them from red-plating. measure unloaded B+ as well.
-30v min, -57v max
BTW, no offense intended, but did you strap pin8 to pin1 on the EL34 sockets?
Yes
/Leevi
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It looks like you have already figured out how to make your current OT work with the EL34s. :worthy1:
I'm sure with a little help from your friends here, you will get that bias issue squared away.
If you or someone else is up for the challenge, I've got some premium odd-ball output tubes that I think could make a great guitar amp.
They are Amperex and Mullard 7534s or E130Ls. A PP pair of these can put out 60W.
I'd be willing to send a pair of these to someone who could design a rocking guitar amp circuit around them and share it with me.
I don't have the skills necessary to do that yet. :sad2:
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what do you measure at the wiper of the pot (node 5) with the pot at max and pot at minimum? <<< pull the power tubes to keep them from red-plating. measure unloaded B+ as well.
-30v min, -57v max
BTW, no offense intended, but did you strap pin8 to pin1 on the EL34 sockets?
Yes
/Leevi
seems out of spec. check the caps c40 and c41. one of them isn't doing the job. you'd expect -60V there with the pot at minimum with 160VAC supply.
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PT = hammond 185G230 FWBCT w/ SS 4A bridge rectifier assembly.
use the 1.8K 100W marshall OT.
schematic of PS concept attached. i built a similar supply for this (http://el34world.com/Forum/index.php?topic=12226.msg115884#msg115884) amp.
start another topic?
--pete
EDIT: minor drawing corrections and some resistor wattage changes.
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> -25V on the grid.
> current is 20mA
About as expected.
Nevermind the plate voltage. The screen voltage here is quite low, 275V.
My plot says at -25V you should get 11mA per tube. (I won't worry about 11 or 20 because we are way down at the bottom of the graph.)
I think you want to be able to trim to -20V. This gives (nominal) 38mA at 250Vg2, probably 42mA at 275Vg2. 42mA at 500V plate supply is 21 Watts Pdiss, which is near or above the "70% rule".
As DL says, be *sure* the parts you have are working right.
A particular oddity: the 6 Watt lamp L1, fed through 4.7K resistor. Is it really there, working? 6W 220V would be about 8K Ohms if it were full-hot. I suspect it is not full-hot, probably dim glow. This 8K with the 5K reduces the AC we get from the "160V" winding, which may not "feel like" 160Vrms because of the many diodes in there. We can make it work with or without L1, but say which way you want to do it and stick with it.
I do not like either trimmer. When the wiper loses contact (we all do that when old) the tubes go to zero-bias and melt-down.
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The capacitors of the bias circuit were fine.
I replaced the 22K resistor (R110) with 68K and got a range for right bias setting.
Thanks for your responses
/Leevi