Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Duncan on April 16, 2025, 09:22:40 am
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Hey gang,
I THINK I know the answer, but I wanted to consult with the hive mind to confirm.
As is my MO apparently, I'm building a circuit inside a gutted Traynor YBA-1. Every one I've done before, I've either replaced the transformers, or built a cathode-biased amp (so I've never needed to use the 60v windings on the stock power transformer).
The wiring diagram shows a separate winding, terminating with two leads pumping out 60v @ 20mA. In previous builds that have used a bias tap, it's only been one lead.
So to confirm, where the schematic says "BIAS TAP," I would connect both leads to that point? Or would I put one lead to that spot, and one to ground?
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Connecting the 2 wires at a single point short-circuits the transformer's secondary windings. Don't do it !!
Picture :
From the transformer secondary 2 wires ; 60 volts 20 ma
One wire to the bias rectifier
One wire to amp ground.
You may have to replace the 22k resistor?
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Picture :
From the transformer secondary 2 wires ; 60 volts 20 ma
One wire to the bias rectifier
One wire to amp ground.
You may need to adjust / change the 22K resistor .
Amazing. Thank you!!
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Picture :
From the transformer secondary 2 wires ; 60 volts 20 ma
One wire to the bias rectifier
One wire to amp ground.
You may need to adjust / change the 22K resistor .
Amazing. Thank you!!
Is that the answer you think ?
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Picture :
From the transformer secondary 2 wires ; 60 volts 20 ma
One wire to the bias rectifier
One wire to amp ground.
You may need to adjust / change the 22K resistor .
Amazing. Thank you!!
Is that the answer you think ?
It confirms one of my two suspicions, yeah!
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Picture :
From the transformer secondary 2 wires ; 60 volts 20 ma
One wire to the bias rectifier
One wire to amp ground.
You may need to adjust / change the 22K resistor .
Amazing. Thank you!!
Is that the answer you think ?
It confirms one of my two suspicions, yeah!
But there were only 2 choices. :laugh:
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Picture :
From the transformer secondary 2 wires ; 60 volts 20 ma
One wire to the bias rectifier
One wire to amp ground.
You may need to adjust / change the 22K resistor .
Amazing. Thank you!!
Is that the answer you think ?
It confirms one of my two suspicions, yeah!
But there were only 2 choices. :laugh:
lol that’s true. I called Hammond and the person I spoke with told me that the 60 V winding is independent from the high-voltage whining, and that both lead would go to the diode.
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I called Hammond and the person I spoke with told me that the 60 V winding is independent from the high-voltage whining, and that both lead would go to the diode.
He probably meant either lead (but not both ends together)
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I called Hammond and the person I spoke with told me that the 60 V winding is independent from the high-voltage whining, and that both lead would go to the diode.
He probably meant either lead (but not both ends together)
Today, when you talk to someone on the phone, you always have to doubt their competence. There are so many call centers served by non-technical staff. And it doesn't matter what kind of company it is.
Duncan may also have misunderstood.
Duncan would do well to understand how a bias circuit works, because a mistake can lead to damage to the power tubes.
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I called Hammond and the person I spoke with told me that the 60 V winding is independent from the high-voltage whining, and that both lead would go to the diode.
He probably meant either lead (but not both ends together)
Today, when you talk to someone on the phone, you always have to doubt their competence. There are so many call centers served by non-technical staff. And it doesn't matter what kind of company it is.
Duncan may also have misunderstood.
Duncan would do well to understand how a bias circuit works, because a mistake can lead to damage to the power tubes.
While not an expert, I do understand how a bias circuit works. It’s more that I’m not familiar with transformers with an entirely separate winding for bias. Probably a bigger gap in “transformer windings” but I’ll fill that when time allows
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For me, the simplest and easiest part of an amp is the transformer.
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For me, the simplest and easiest part of an amp is the transformer.
Yeah, normally it’s pretty plug and play, but when you’re used to one or no bias leads, it made me pause and go, hold up what
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For me, the simplest and easiest part of an amp is the transformer.
Yeah, normally it’s pretty plug and play, but when you’re used to one or no bias leads, it made me pause and go, hold up what
I mean , when is time to fix an amp, the easiest part to see how it should work and if it work , it is the PT.
Other circuits in the amp are more complex.
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I'm actually getting ready to replace the PT on my 68' YBA1 due to shorted heater windings.
I have been running on EL34s for 10+ years, never been sure that the stock PT was rated for but seemed popular and safe enough thing to do.
Now I'm not so sure.
Is that the official diagram for the 291JX?
Only 2.5A for the heaters?? Is that right?
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I'm actually getting ready to replace the PT on my 68' YBA1 due to shorted heater windings.
I have been running on EL34s for 10+ years, never been sure that the stock PT was rated for but seemed popular and safe enough thing to do.
Now I'm not so sure.
Is that the official diagram for the 291JX?
Only 2.5A for the heaters?? Is that right?
Yeah, that's the official diagram from the Hammond site.
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I'm actually getting ready to replace the PT on my 68' YBA1 due to shorted heater windings.
I have been running on EL34s for 10+ years, never been sure that the stock PT was rated for but seemed popular and safe enough thing to do.
Now I'm not so sure.
Is that the official diagram for the 291JX?
Only 2.5A for the heaters?? Is that right?
Yeah, that's the official diagram from the Hammond site.
That leaves out EL34, KT77, KT88 😨
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running on EL34s for 10+ years
Is that the official diagram for the 291JX?
Only 2.5A for the heaters?? Is that right?
Each EL34 draws 1.5A heater current, so…
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running on EL34s for 10+ years
Is that the official diagram for the 291JX?
Only 2.5A for the heaters?? Is that right?
Each EL34 draws 1.5A heater current, so…
Yeah, no kidding! 😳
YBA1s after 1970 or so came with EL34... My earlier model came stock with 7027s.
So for the op, myself and many others have we been running them with EL34s, overstressing the PTs this entire time??
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It would appear the "best" drop-in replacement for the YBA1 would be the 273BX, although slight narrower should be +/- the .063in that the mounting tabs allow play for.
That one is 700v DC CT, plus 5A AC of 6.3v.
350/350 = 490v of very healthy DC.
You get about 50v more to the plates than stick 440ish, but if you're gonna be running EL34s most if not all should handle no problem. Use a higher B+ dropper like Marshall if you need to brown it out a bit more.
Much cheaper too.
Huge funk-up on Hammond for advertising the 291 as a "drop-in replacement?" Heck no.
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In the schematic in opening post - Cap polarity is incorrect.
Bias ckt is meh.
See attached - Bias winding is not a tap - It's a full winding. So, if you're using a 1/2 wave, ground one end of the winding.
--Pete
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In DummyLoad’s schematic, the 60VAC at the bias winding will rectify to -84VDC (at the pot input end of the pot/tail resistance) with about -54VDC at the junction of the pot and the tail resistor (With bias diode and filter cap configured for negative bias voltage). Depending on what your desired bias range is, you might want to decrease the (22k) tail resistor somewhat, or use a 25k or 50k pot. Rule of thumb is the bias voltage range wants to be around 10-15V either side of 10% of the plate voltage for the tubes you’re using (so if the plate voltage is say 460, 10% of that is 46V so aim for 36 to 56v adjustment range - negative voltage that is)