My 18w TMB Kit Build

joulupukki

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New gut shots with the Heyboer 18w Power Transformer installed. For now I didn't trim the PT wire leads and just zip tied them to keep them somewhat tidy. With the 150R cathode resistor that I had previously installed, I measured that it's running the output tubes at around 82% plate dissipation.

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joulupukki

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Is it my imagination or does it actually break up earlier than it did with the Antek Toroid PT? Maybe at some point I'll put the original 130R cathode resistor back in and see how it handles that but it does seem like it goes into overdrive a lot sooner on the volume knobs. It still seems to sound pretty good.
 

2L man

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Combined Cathode current comes 76,8mA (~38,4mA each) so Screen currents idle at about 8mA each which seem highish. If your amp has individual screen resistors measure voltages over them to calculate Screen current that way too.

I have build only one EL84 amp and I don't know that tube and it might be typical?

I always install 1 ohm resistors to each power tube cathode and checking bias current accurately is easy. OT primary resistance work but after voltage loss is measured the amp should be turned off and OT resistances must be measured again because OT temperature has effect to them.
 

King Fan

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Is it my imagination or does it actually break up earlier than it did with the Antek Toroid PT?

If you cool the B+ or the bias it is often said to warm the tone but decrease the clean headroom. Is this the amp that was biasing over 100% at one point? Can you remind us about B+ and MPD on the old PT but with this bigger 150ohm resistor?
 

joulupukki

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If you cool the B+ or the bias it is often said to warm the tone but decrease the clean headroom. Is this the amp that was biasing over 100% at one point? Can you remind us about B+ and MPD on the old PT but with this bigger 150ohm resistor?
Yes, it was over 100% with the Antek PT and a 130R cathode resistor so I changed out the 130R for a 150R and it calmed things down a bit (took it to 87% MPD). B+ with the toroid was 342V.

Now that I've got the actual PT meant for this, it's chilling at 82% MPD and yeah, definitely less clean headroom.
 

joulupukki

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I always install 1 ohm resistors to each power tube cathode and checking bias current accurately is easy. OT primary resistance work but after voltage loss is measured the amp should be turned off and OT resistances must be measured again because OT temperature has effect to them.
At one point I had installed some temporary 1 ohm resistors and before then even tried the shunt method of measuring the actual current with my multimeter. This way that I'm using now to measure and calculate is surprisingly very close to those actual measurements so I haven't bothered putting the 1 ohm resistors back in.
 

joulupukki

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I put the 130R cathode resistor back in to see how hard the power tubes would be driven with the Heyboer PT. Here's how it's looking. I used the shunt method to measure current directly.

V4V5
Plate Current37.9 mA37.4 mA
Plate to Cathode Voltage325 V325.7 V
Plate Dissipation12.32 W12.18 W
Plate Dissipation %102.6 %101.5 %
 




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