Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: creekrat on February 21, 2017, 10:53:47 am
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I am looking at using a hammond ao-43 to make a matchless clone and am looking at using the trinity tc15 schematic and layout. The ao-43 PT is 315-0-315 which i know will be high on the B+ for the EL84 power tubes and the trinity schematic calls for 290-0-290. The circuit also calls for a 5AR4 in the rectifier. If i swapped the 5AR4 for a 5Y3, is there a way to calculate the theoretical voltage drop on the B+?
I guess the first question would be where exactly on this schematic would you measure the B+?
So would a 5AR4 that has a voltage drop of 10 bring the trinity spec'd PT to 280-0-280 and the hammond to 305-0-305? Is that how that works?
Sorry if some of these questions seem simple but I am far more of a visual learner.
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I think you will struggle to reduce the voltage on that AO-43 PT. I used an AO-43 as a donor for a Plexi 6V6. My PT puts out 346-0-346. Using a 5AR4 my B+ was 400VDC at the first filter cap. If you bias the Lightning hot you may drop that 400v another 10 volts. And using a NOS 5Y3 should drop even more.
I built a Lightning with a 275-0-275 PT and a 5AR4. B+ was 336VDC at first filter cap.
So would a 5AR4 that has a voltage drop of 10
You can't just say a 5AR4 has a voltage drop of 10. The voltage drop across a 5AR4 (or any other tube rectifier) is dependant upon the current flowing through it, ie, the load. You can say that a NOS 5Y3 will have a greater voltage drop than a 5AR4, but without knowing how much current will be flowing, you cannot say how much that voltage drop will be.
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I have an el84 Traynor Bassmate. Stock 410V plates. I got it down to under 300V using: dropping resistor to drop about 50V & a 12V bucking tranNY. If you want to use that PT you have options .
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> where exactly on this schematic would you measure the B+?
Why do you care?
1) At the filter-caps, so you buy a good Voltage rating
2) At the power tubes, to see if they are OK.
315V AC will give a start-up surge near 315V*1.414 or 441V. 450V caps are nominally fine. This will quickly drop as the power tubes start to suck.
There is no significant voltage-drop from first cap to EL84 plates, and 440V is awful high for EL84. We need to investigate rectifier drop.
Rect drop depends on load current. 90% of the load in a power amp is the big bottles, the EL84s. We find "8.3V across 120r" in their cathode return, so 69mA, call it 75mA with little tubes.
GZ34 is absurd for a piddly little 75mA. As you say, 5Y3 is right-size.
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/5/5Y3GT.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/5/5Y3GT.pdf)
Bottom of Page 4. 300V AC per plate at 75mA is around 340V DC. 315VAC is likely 5% higher, say 357V. Deducting 8V cathode voltage, 350V on tubes.
_I_ would not fret about 350V on any new-made "EL84". I think the "300V" rating was conservative, to cover cheap plate-stuff when mass produced for car radios. As their market moved to guitar amps, EVERYbody was over-volting them, better plate stuff was used to control complaints. They tried charging more for a "7189" with 400V rating, but only the hi-fi market took that up. Today they don't make enough to bother with cheap plate stuff. I did not think thrice before shoving "EL84" in a Traynor BassMate at 410V. Even though I would have to fix it if they blew-up.
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Hi
I've built deluxes in A0-43s but admittedly I'm not too familiar with the Matchless
Would it spoil the design to build it with 7189s?
You'd have a slightly louder amp but you'd not have to worry about your B+
The 7189 is a good sounding tube, same 9 pin tube socket and pinout (I think the 7189a is different)
Is the AO-43 still together?
Can you power it up with tubes and try a 5Y3 in it and then test the voltage at El84s/6BQ5
(maybe yank the 12BH7)
That would give you some idea