Mini-Bassman question

[bmccowan]

I recently had the happy experience of finding in my stash a PA conversion I started, never finished, and had forgotten about. I had started a Geezer mini-bassman. So I looked at all the things I had done wrong 15+ years ago and got it up and running. I followed the Geezer schematic with the exception that I reused the PA power transformer and an EZ-81 rectifier - voltages are good. OT was bad so I used one from a Hammond AO-44. It sounds great! My question has to do with my expectations for amps with this type of master volume. I mostly work with lower gain amps with no master volume. I have no need or reason to dime either the gain or the MV, but what are the experiences out there? I find that the MV around 5 gives a very useful range of the gain pot. MV much above 5 and I have to be pretty careful with the gain pot. I find the same with a TMB 18W I built.
BTW - love those 6BM8 tubes. In addition to this amp I have a Gibson GA-8T and a HoSo-56 minimalist - love em all. I've seen complaints about the cost of NOS 6BM8s but I have found great RCA and Mullard pairs at reasonable prices.

[uki]

I have one amp about 40w that have the MV, usually its dialed to about 3-4, and gain about 2-3, gives a very clean tone at low volume. When doing gigs, the MV is dialed to 5-6 and above, depending on the acoustics, the tone gets more warm, with the MV at 9, and gain at 5, if middle control is pushed the PA tubes start to overdrive. But I do really prefer amps with just the pre-amp volume controls, it feel like the MV when down does modify a lot the general tone and power of the amp.

The more MV , easier the gain control will make the amp drive, specially if you have mid control, the more you push the mids, more drive !! Try low MV , high gain, low mids, might give more clean tones.
Bass control also may add some drive.

Check this out: https://el34world.com/Forum/index.php?topic=21734.msg230910#msg230910

[bmccowan]

Thanks Uki - that is a great thread, especially the advice from Tubenit who is always ready to help. He gave me advice for a very minimalist version of the HoSo56 that worked out great. It seems that this mini-bassman has more gain on tap than I will ever use. Too much preamp gain makes it difficult to get good power tube break-up. But what do I lose if I dial back the gain at V1? - experiments likely to follow despite the fact that the amp sounds great.

[tubenit]

Some thoughts to ponder.  I am not saying your should do any of these, just tossing stuff out to consider. Alot of the overdrive tone comes from the "mids" being accented.  I usually find a mid-scooped amp to sound cleaner (as though it has less gain).

Use a 12AY7 in V1.
Use a 12AT7 in V2
Increasing the V2a cathode resistor from 1.2k to 1.8k should allow for a somewhat cleaner tone.
Using LNFB  (local negative feedback) gives a cleaner/clearer tone.  This can be put on a mini-toggle.

Changing the tone stack.  I have always wondered if the .0047 bass cap in the tone stack was a typo?  Increasing both the bass cap to .047 and perhaps increasing the midcap to .03 will give you a more midscooped tone which will sound cleaner.

You may find a PPIMV more useful than the traditional pre-phase invertor type master volume?

IF I were going to experiment, my starting place would be:

1)  12AY7 in V1
2)  12AT7 in V2
3)  .047 bass cap in tone stack
4)  V2a cathode resistor increased to 1.8k


With respect, Tubenit

[bmccowan]

Thanks Tubenit - I agree on the mid-scoop and like this amp with the mid pot all the way down or on 1 with treble and bass in the 3-6 range. I do have a 12AY7 in V1 - did not mention it as I assumed it was standard in V1. I tried a AX7 there and like the Y better. I tried an AV7 in V2, will try an AT as you suggest and will definitely try the cap and resistor changes one at a time. I will save changing the MV for last. I used that on a 6V6 Clubman variant and liked it.

But I think you can also help me understand the practical limits of the interacting gain and MV, as almost all my experience is with lower gain amps with no MV. And this question is academic as I do not want to play the amp with the gain and volume turned way up, but as I reach the upper limits on both at the same time, both on 8-9 or so, with P-90s, the amp gets unstable - feedback - my dogs hide - so does my wife. I know Sluckey would say "well don't do that" and I won't/don't, I am just curious if I am experiencing the typical practical limits. The amp sounds great with a quiet noise floor below those levels.

[tubenit]

Almost every amp I've built would likely be unstable or simply sound awful (IMO)  if all the pots were at 9 or 10.  Typically my volume on amps is between 3-5 (usually around 4) and seldom above 6-7.  I usually have treble around 5-6, bass about 7, mid about 3.   If I have a PPIMV, that's usually 6-10.

I like some sustain and harmonics with the notes just on the edge of overdrive or somewhat past that line. Just a personal preference.  And I don't care for amps that sound muddy where I can't hear the note separation well when chording.

Leon C's  Supro 1688TN video is about as close to an ideal tone for what I like as any video I've listened to.  Larry Carlton would be another good example


Supro 1688TN Based Homebrew Amp Demo by Leon C - YouTube


The Guitar Gods - Larry Carlton - "Blues Force" - YouTube



With respect, Tubenit

[bmccowan]

Thanks Tubenit. I sometimes think I waste time chasing down noise and feedback at volume settings I never use. Just trying to get the amp "right." For these MV amps I think my expectations have been off-base.
Totally agree on that Leon C vid and have sent many people there. His "Smoothie" is another favorite of mine.
I need to listen to that Larry Carlton tune.
Many thanks - you are always willing to help.

[bmccowan]

I just replaced that .0047 bass cap with a .047 as you suggested Tubenit, and played the amp for about an hour. Love it. It is really a great sounding amp at reasonable home volume levels, and I have no urge to start plugging in pedals.
So I have V1 12Ay7 - V2 12Ax7. I forgot that I also previously added a 33K grid stopper for V2 - it did not do much IMO, but I left it in. I added a switch to turn off the NFB and I like that at low volume. Cool amp. For interested tube sniffers, I have both RCA and Mullard 6BM8/ECL82s - they sound the same to me.
BTW - On Leon C - that dude can play guitar! He's got the touch. I wish I could get there.