Master Volume for Fixed-bias Single Ended 6BM8 amplifier ?

[walkman]

Hi All.

While, most single ended guitar amps are cathode bias, i have converted an old mantle radio to a guitar amp.
Happy with the sound and tone but it is still too loud for home use though, as you might understand.

I have been searching the web for an example of a SE guitar amp that uses fixed bias and has a master volume.

Any suggestions (other than convert it to cathode bias) or examples of how to do this?

Cheers

Walkman

[tubenit]

Some questions for you:

-  why fixed bias instead of cathode biased?  What's your thinking on what that would accomplish? 

-  you talking about changing something on the radio to guitar amp project?

-  have you got a schematic of what you've done that you can share?

-  is the goal to modify your project or simply come up with a lower volume amp that sounds great?  IF so, we can offer
   some ideas for you.

With respect, Tubenit

[tubenit]

IF the primary goal you have is a good sounding low watt amp with a master volume ………….

Here is an example of a low watt amp using pentode/triode tubes  (ECL84 pair is about 3-4 watts) & it has a PPIMV master volume.   The push/pull allows some "hum-cancelling type characteristics" that single ended amps don't have. The ECL84 tubes are inexpensive.  Since you're already familiar with pentode/triode tubes, this might be a consideration for you.   Only 3-4 watts with a master volume.

I attached an editable SCH version of this schematic also.  The attached schematic is a "keep it simple" version of the SoLow Watt that is a proven amp design.  Here is a great thread about someone who built a Bumble Bee/SoLow Watt:

http://el34world.com/Forum/index.php?topic=13151.0

This SoLow Watt idea was built by several guys on the Amp Garage also & they reported liking it and thinking it had remarkable bass response for a small watt amp.

NOT saying you should do this, simply illustrating a idea for you to consider since volume seems to be an issue.

With respect, Tubenit

[tubenit]

This would be an example of a singled ended amp with a master volume.  The master volume is the level pot of the active FX loop. 

Here is a sound clip of the amp:  https://www.soundclick.com/music/songInfo.cfm?songid=13752309

With respect, Tubenit

[shooter]

examples of how to do this?

there are a few but you gotta go back to the '40s era.

by nature an SE wants to run at / near Max plate or more, the harder you beat it, the "cooler" it becomes (basically).  I would highly recommend leaving xSE self biased for too many reasons and PP either way you want.
"master volume" in SE is real easy, just put in at the input to the PA driver, walla 

[walkman]

Hi Tubebit and Shooter,

Volume is an issue unfortunately! Being an Aussie amp man I have one of Darryl's Valve Tone impacts with the ECL84 and it is too loud for family and next door when cranked.

Hats off to all those on this and other forums that do inspiring and excellent builds and designs and answer questions for beginners like myself. I enjoy building and understanding how to get the sound i'm looking for.

The amp in question is already built and sort of a prototype / steeping stone to another build i did after that. I have collected  a number of cheap or unwanted valve radios and grams and have in mind that i can put together some designs that others can follow based on my efforts and the parts available in this reasonably available equipment.

So the goal  of the amp was to have a clean amp up to about  7 on the gain, with a mid control to get some mid scoop.

Having moved since the amp was initially built, a master volume after the preamp is now essential to bring the volume down for home friendly levels.

The doner radio in this case was a kriesler 11-80 https://www.kevinchant.com/uploads/7/1/0/8/7108231/11-80.pdf






The way i have setup the pentode section of the 6BM8 is actually combination bias, with a small cathode resistor. My reasoning being that i could use a cathode cap then to introduce a bass boost  as required. Though i haven't tried lots of different values for the cathode resistor  and cap i'm happy with the value i've settled on.

The question I have is how to put in a master volume?, if it were cathode bias only, it would simply replace the grid leak. Would it be best to do it like half your SoLow Watt? before the bias resistor? are any additioal caps or resistors needed to keep DC off the pot ? Will the loads be in parallel, requiring consideration of the values so to keep the same resistance to ground at max volume ? What do you suggest?




PS. I'm naming this BC 4, B for BroadCast and 6BM8, 4 for the approximate output at overdriven tones. Maybe it could be called the BBC 4 

[tubenit]

I'd probably experiment with something like this given your parameters.

With respect, Tubenit

[walkman]

Things move slowly.... however finally have reworked the chassis ( removing the original front plate) and build rough but robust head shell. Master volume here we come (at snails pace)

[walkman]

Master volume in. Works well thanks tubenit!

[tubenit]

Thanks for sharing your success with the master volume!

With respect, Tubenit