Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BluegrassDan on October 15, 2021, 10:35:53 am
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I have an original 1959 Fender Bassman 5F6A on the bench. New caps, tubes, etc.
The B+ voltage has me on edge. Throttling the wall voltage to 117 VAC with a variac presents a B+ around 470vdc. Plugging directly into the wall at 123 VAC is pushing the B+ to up around 500vdc.
I can’t fathom replacing the original power transformer with a new lower voltage version. It also seems risky to go with a different rectifier tube that may draw more heater current.
Does anyone have advice on how folks are handling the higher modern voltages? I am using NOS Tung Sol 5881s. I know they are tough, but I don’t wanna fry them.
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Voltage itself means nothing without current.
What bias current measurement do you have?
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Increasing the bias resistor from 56k to 75k gets it to around 32mA (65% plate dissipation). Of course, that pushes the plate voltage up a bit more to around 495v.
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I just wanted to make sure you had the tubes in when you were measuring voltages.
Yes, those voltages seem high, BUT, 90% of tube amps are run without regard for maximum voltage specs.
It's a give and take. If you don't push it, the tubes will last longer.
If you do push it, don't get attached to NOS tubes.
Modern 5881 have a 500V max. plate V limit which is just there to remind people to not apply 700V.
You could build a bucking transformer and knock the input voltage down a bit, and just leave it in the cabinet.
PowerPoint Presentation (wordpress.com) (https://blueglowelectronics.files.wordpress.com/2018/11/bucking-transformer-slides.pdf)
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The heater voltage (loaded) is a good metric of the intended primary voltage.
If the 6.3VAC heater supply is running up near or past 6.9V, then consider a mains bucker or some other means of reliably throttling it back.
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Sluckey has information on building a bucking transformer.. second item here http://www.sluckeyamps.com/misc/Amp_Scrapbook.pdf
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After some more tweaking, I'm resigned to a poorly matched pair of 5881s. They don't buzz, but one is drawing only 27mA while the other is drawing 35mA at 280 VDC plate voltage.
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.... but one is drawing only 27mA while the other is drawing 35mA at 280 VDC plate voltage.
280dcv?
That's not that bad on the matching, 3 or 4mA is very acceptable.
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8mA
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Build a 12v bucking transformer and you’ll be good to go.
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.... 280 VDC plate voltage.
I assume this is a typo.
Before you were worried about 470V on the plates.
Suddenly you lost 190V? In the amp or in your typing fingers?
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Typo! 483vdc to be precise.
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Is the amp working now?
All modern "6L6" (mostly 6L6CG spec) are good for 500V; only the 1937 era production had to be de-rated down from 400V to 360V.
The 8mA (really 1:1.33) current difference is IDLE, on an amp designed to suck MUCH more current when loud. Peak current in this amp is near 300mA. 8mA difference, pah! close enough to rock. It does not limit your maximum power.
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That's not that bad on the matching, 3 or 4mA is very acceptable.
8mA
Yes, what I meant was 3mA to 4mA is very acceptable, then 8mA is not too bad.
Have you flip/flopped the 2 tubes from 1 socket to the other socket? Sometimes you can pick up a few mA's so their closer to being the same in current draw.
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Does anyone have advice on how folks are handling the higher modern voltages? ...
Most are buying variacs or the AmpRX BrownBox (https://www.amprx.net). I personally use an old variac (that another member here sent me) to power my vintage amps as they take different voltages.
You can build a bucking transformer to have a custom solution for one amp, but you'll be back where you started when the line voltage changes.
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Clearly, this amp was made for 110VAC wall voltage. At 110, all of the voltages are spot on with the 5f6A schematic. The tube matching is less significant at this normal voltage.
Since this is for a client, I may just indicate in writing that this amp should be used with a voltage regulating device such as BrownBox. Also looking into building an in-line bucking transformer box.
Thanks for the help, everyone.
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You can build this small self-contained unit for about $20. See page 2...
http://sluckeyamps.com/misc/Amp_Scrapbook.pdf
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I have considered this upon learning about it.
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I have build Sluckey's bucking transformer, works like a charm, off, 7volts dropped or 13 volts.
And small enough to fit inside the Bassman.
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BluegrassDan, I pm'd you a transformer suggestion. It's just a source for the RadioShack model some of us have used.
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Thanks so much for all the helpful input.
I also noticed this guy: https://www.apc.com/shop/us/en/products/Line-R-600VA-Automatic-Voltage-Regulator/P-LE600 (https://www.apc.com/shop/us/en/products/Line-R-600VA-Automatic-Voltage-Regulator/P-LE600)
It has settings for regulated 110v, 120v, and 127v output.
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I also noticed this guy: https://www.apc.com/shop/us/en/products/Line-R-600VA-Automatic-Voltage-Regulator/P-LE600 (https://www.apc.com/shop/us/en/products/Line-R-600VA-Automatic-Voltage-Regulator/P-LE600)
As a performing guitarist, I would want the LEAST amount of circuitry between my amp and the wall receptacle, so I would opt for the transformer in a box rig.
When I built mine I left out the switch in sluckeys design and just wired it to lose 12V
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I'm wondering if this 16.4V transformer I salvaged from a Fender Frontman 15R would work? Maybe wired without a switch for 8V down? That would knock my 123VAC wall voltage down to 115, yes?
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I'm wondering if this 16.4V transformer I salvaged from a Fender Frontman 15R would work? Maybe wired without a switch for 8V down? That would knock my 123VAC wall voltage down to 115, yes?
What's the current rating?
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Our mains voltages here in Memphis are 122-123v.
I built a couple bucking boxes and used Triad F6-10 transformers. 10VCT@3.0A
I get 110v or 117 with them.
I used ATX computer power supply cases to house them. Already has a IEC power jack.
http://catalog.triadmagnetics.com/Asset/F6-10.pdf
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What's the current rating?
That's a good question. I can't find any info online, only the schematic:
https://el34world.com/charts/Schematics/files/Fender/Fender_frontman15g.pdf
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What's the current rating?
That's a good question. I can't find any info online,....
It's 15 Watts audio output. So at least 25 Watts of DC to the final stage.
The DC is + and - 23V. This is consistent with 16V of AC on each side of the winding. Total 32VAC. (Not the 8V and 16V you say.) This suggests 0.78 Amps of AC.
There's no way you want 32V of drop as a normal thing. You can use one 16VAC side to drop (say) 125VAC to 109VAC, a pretty wicked brown-out.
At 0.78A this would support a nominal 93Watt load. But half the winding is idle so we can put a little more through the winding we are using. So maybe a 5F6a.
You can find much more suitable iron. Getting it delivered in the current market may be another thing.
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Thanks PRR. I really don't have an immediate need to drop voltages; I just had that TX sitting on a shelf and wondered if it was good for anything.
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I decided to build a "slucky," using a Hammond transformer housed inside a 4.74" x 3.95" x 2.37" die cast box. It will have a fuse holder and 1' lengths of cable with strain reliefs. Not switchable, but set to -12.6v.
Bam! Thanks, y'all.
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Which Hammond transformer?
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187E12
56VA, 12.6VCT, 4A
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I may need to build more of these and I wanted a transformer that was in production currently.
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I may need to build more of these and I wanted a transformer that was in production currently.
Radio shack no longer makes the PT in sluckey’s post but several ate on ebay all the time #273-1511 search ebay
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I had until fairly recently a very bone stock genuine 1959 5f6a chassis. I ran both vintage Tungsol and Reissue TungSol and both vintage Mullard GZ34 and Reissue TungSol 5AR4 rectifier,,,,, all at regular wall voltage fairly regularly. This amp was nearly 98.7% all original components including original filter caps for nearly six months. (albiet the caps had been meticulously reformed over the course of several days) Eventually the filter caps were replaced but the amp was regularly auditioned outside of my shop at whatever wall voltage happened chance be, within the SoCal Edison Grid.
Maybe Ignorance is Bliss but never a problem.
However, at my shop location where the amp usually resided my power entity is provided by the municipality. For better or worse? I can't really say.... But the amp was auditioned daily at the shop and often would be cranked up to its full singing glory.... at this location I utilized my variac/auto/iso transformer and would sag the amp at around 100v for some awesome tonal tweed goodness!
But yeah no prob higher wall voltage with all orig factory components.
My only question is what the hell possessed me to sell that amp?!?!
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Operated at wall Voltage......
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Clearly, this amp was made for 110VAC wall voltage. At 110, all of the voltages are spot on with the 5f6A schematic...
In the absence of the previously requested heater voltage reading, I wonder whether you may be jumping to a false conclusion?
The only certainty is that the 5F6A schematic and layout is applicable to amps made around Sept 1957. And even at that time, 117V appears to be the intended mains supply voltage.
After that, the design of the amp probably evolved, and by 1959, that info may not be totally representative to the Bassman then being made.
https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a.pdf
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Ta da!
Pretty simple plug and play. Single voltage attenuation, not switchable.
Thank you all for the great idea and suggestions.
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Just a note on the 115vac to 10vct 3A transformers.
The , Thordarson CFP 501 , Triad F6-10 and the Hammond 187D10, are basically the same design.
All feature the lugs that accept quick connectors.
Mouser has has the , Hammond 187D10 :icon_biggrin:
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...basically the same design. ...
Signal 2-4-1, 1970s.