Bassman 5F6A

[rymac]

I want to know in a bassman 5F6A what is the ideal voltage at the the 1st preamp tube plate? Is this measured before or after the resistor??

RyMac

[Chuck H]

Measure after the resistor (at the tube socket). If you want to know the voltage on the plate of that tube, you have to measure it at the plate.

"What is the ideal voltage?" is a trick question because there is more than one answer. An ideal 5f6a Bassman ran on 110V AC mains and should have had about 150V DC on each plate of the first preamp tube. But if you plug that same amp into the wall today using 120V AC mains you should get about 180V DC. So if you are building a clone amp and want the vintage spec to be your actual voltage then you want about 150V DC. But if you have an actual 5f6a Bassman and you want to know if it's operating correctly then you want to see about 180V DC.

In the real world I think most clones fall somewhere in between.

Chuck

[rymac]

Thanks for the help
I have a Twin Reverb that I rebuilt into a bassman.
I Changed out both transformers for a 40 watt 2 6L6 amp. I built the power supply more like a new Fender. I'm not trying to make a bassman clone. I measured closer to 200v DC. I'm I missing out on something tone wise with this higher voltage. I know the fender bassman reissues had higher voltages than the originals. I also know a lot other things add up to make great tone (tubes, speakers, quality parts etc...)
I like the sound of the amp but like to tweak my gear. Sometimes it's hard to just leave it alone.

rymac

[Chuck H]

It's an easy experiment to see if your missing out. You only need to change the B+ string dropping resistors for the preamp plates. So, for example, if you have an arrangement like :

Main filter-choke-screen supply filter-4.7k R / PI filter-10k R / V1 & V2 filter

You could try a larger resistor behind the PI filter. So instead of 4.7k you might try 10k or even 20k. This would drop the voltage on the PI and preamp. If the PI voltage is to your liking you could change the resistor behind the V1 & V2 filter from 10k to 20k. This would drop volts for V1 and V2 only.

You could experiment with values until you get the "vintage" spec or you may find something you like better in the process.

It's worth noting that increasing the resistor values here will increase "sag" in the following gain stages. But so little you may not notice.

Chuck

[MWJB]

"I measured closer to 200v DC." That's on a 12AY7 right? If using a 12AX7 225-235v is typical.

[rymac]

OK I didn't know the type of tubes would effect the voltages.
I have used a 5751 and 12AX7 at different times in the V1.
I have always used a 5751 in the V2 spot.
I'm also only using one side of the V1 tube. I connected pins 1, 2 and 3 to ground. Will this effect voltage??

I'm going to start over with measuring my voltages.
I will put a 12AY7 in the V1 and a 12AX7 in the V2.
That seams to be the standard.

[rymac]

I also wonder what difference in voltage you would see in tube vs solid state rectifier.

[MWJB]

"I will put a 12AY7 in the V1 and a 12AX7 in the V2. That seams to be the standard." 12AX7 is the most common tube that you will find in V1, a lot of guys will find 12AY a bit thin/clean unless you're using a hollow body guitar/big output pickups.

In a 5F6A you'll typically see about a 5V drop in B+ when switching from SS to 5AR4.

[rymac]

Just an update. I changed some resistors in the power supply.
I used Tung Sol 5881 , solid state rectifier and set the bias at about -33mA.
I use the transformer shunt method.

V1 plate 12AY7 168vDC
V2 plate 12AX7 323vDC

It seams to be within range of where I need to be.
Rymac

[Chuck H]

How's it sound ???

[Bernardduur]

Can I add another power filter stage behind the others for an even lower voltage?

I build a blackface bassman and added a 5F6A preamp as the bass channel (found the bass channel a bit to muddy for my taste). The voltages of the unit are OK for a blackface bassman but I noticed that the 5F6 runs on another voltage which is approx 80V lower.


Or is it possible to run another power supply filter parallel to the blackface one only for this preamp?

[Chuck H]

If your planning to add tube/s and a whole other preamp then I would say that adding filtered stages is not just possible, but prefered.

FWIW what you have done is considered "hijacking". If a post or thread gives you an idea or promts a question it's usually best to start a new post.

Chuck

[Bernardduur]

Sorry for the hijacking!

[rymac]

How does the amp sound?

I played it at an outdoor gig yesterday and it sounded good. I used a 5751 in the V1, 12AX7 in V1 and Svetlana winged C 6L6's.
I'll try a 12AX7 in the V1 at some point.
I tried a carbon comp 56K slope resistor but I I'm going to change it back to carbon film. Can't really explain what I'm hearing that I prefer carbon film. The only carbon comp resistors in the amp are the 68K on the input jacks. I might try replacing them too. I'm using 1K grid resistors on the power tubes and shielded wire with ground connected to power tube ground.

I was able to turn the volume up to 5 and used a SS rectifier. I don't like too much sag. I've tweaked tube amps for a few years and I can't really pass judgement on them until I play them with a band. In your living room is one thing with drums, bass and another guitar player is another. The speakers 2X12 one Tone Tubby alnico and one from the original Twin Reverb Reissue it's over ten years old and seams to be broken in. I may change it out with a Vintage 30 from another amp.

The other guitar player uses a Traynor YCV40, guitars with humbuckers and really good pedals. I play a custom built strat. We seam to blend well together. I'm curious what advice if any people have on playing with two guitars in a band. Gear, setup?? Maybe a new thread.

Thanks for the help. We record next weekend.