easy swap Bandmaster PT into Showman?

[deci belle]

I can't ordinarily drive the 66 showman loud enough to get rid of certain subtle(?) high frequency artifacts on the clean channel. Can I install a bf bandmaster power tranny without other considerations so i can just drive two 6L6s?

At low volumes, the tone is thin, especially with the neck pu playing jazz on a big box. But when cranked, the clean amp tone sounds real good to me.

I've tried pulling two tubes, but the clean channel looses some sparkle that way. The other channel doesn't care as it's an MV 5f6 with 70v on the extra gain stage, and breaks up too easily to hear the high frequency oscillation at any volume to sound objectionable.

I'd prefer only have to buy one tranny with multiple impedance taps if possible.

[Tom Phillips]

The BF Showman and Bandmaster shared the same chassis and circuit except for the transformers and number of power tubes.
Having said that I’m a little confused about your questions. See comments below.
Before you make any changes I suggest that you play a bandmaster and decide it it gives you the sound you want.

I can't ordinarily drive the 66 showman loud enough to get rid of certain subtle(?) high frequency artifacts on the clean channel.

First time I heard that complaint and what do you consider the “clean” channel on a 66 Showman?

Can I install a bf bandmaster power tranny without other considerations so i can just drive two 6L6s?

That will work but there are other considerations and I don’t see how it will address your complaint. You could also just add some series resistance to the HV path to simulate more transformer sag. Without changing the OT you will have an impedance mismatch to your speakers. Does your existing 66 Showman have an 8Ω output impedance (True Showman) or 4Ω (Dual showman. Most of which were marked “Showman”)

I've tried pulling two tubes, but the clean channel looses some sparkle that way.

If that’s the case it may be because of the output impedance change and maybe because of a slight bias change because the B+ rises due to the lighter load. Again, I don’t think that a Bandmaster PT will help solve that but other tweaks might

I'd prefer only have to buy one tranny with multiple impedance taps if possible.

Now you’re talking about the output tranny so you have already mentioned two tranny swaps.
The 66 Bandmaster used a shorter stack PT that fit the same footprint as the Showman PT and a much smaller OT as also used in the Pro Reverb, Tremolux and Vibrolux Reverb the same year.

[g1]

Can I install a bf bandmaster power tranny without other considerations so i can just drive two 6L6s? ...I'd prefer only have to buy one tranny with multiple impedance taps if possible.

Did you mean output transformer? Not sure which Fender's had multiple taps, maybe some of the newer stuff but the older ones just got mismatched when you used the ext.spkr. jack.

[JoeM]

What kind of cab and speakers are you using with the Showman?

[deci belle]

Hi— thank you for all the replies!

Joe, the speaker is the 8Ω D130F 115 that came with the single Showman 8Ω head— it's a '66.

g-one, yes, of course!! That's what I should be concerned with— the OT, not the PT. I wanted to ask about installing a Bandmaster OT in order to drive two 6L6s. Are there other considerations I need to address in order to do this, or is it a simple matter of swapping out the Showman OT?

Hi Tom! I want to drive two tubes instead of 4. Why? So the amp will find its groove at lower volumes. Why? To see if I can get rid of some subtle(?) high frequency artifacts that have been plaguing this rebuild for over a year-and-a-half. I discovered recently that as the amp is cranked (too) loud those certain ghost notes when fretted on the little string seem to go away.

So I want to try driving the amp harder at lower volume to see if the issue of the high frequency artifacts is just a matter of playing an 80w amp below its threshold. That's why I want to install a Bandmaster OT (thanks g-one!).

I apologize for that glaring mistake, Tom— Sorry!!!

[JoeM]

IMO, a transformer swap isnt going to help much, if at all to get a better sound at low volumes. You mention some kind "subtle high frequency artificacts" in the sound. Before doing any swaps I'd be sure there arent any other problems, especially filter caps that may need replacing. But even so, most tube amps do need a certain amount of volume before they get in their sweet spot. You might even try a less efficient speaker that will let you turn the amp up more.

[Jag]

+1 on what Joe just said.

I'd also suggest checking if the amp might be biased a little cold. How is it biased right now and did you re-bias, as was implied by Tom?

FWIW, those amps sound pretty good to me with the stock transformers if everything else is right.

[deci belle]

Um, does anybody know if I can swap in a bf Bandmaster OT into my bf Showman without other considerations so I can push this amp harder at lower volume to test my hypothesis?

Gentlemen, after a year-and-a-half I've got all your bases covered coming and going. I learned how to deal with a scope in the process. Please save the troubleshooting suggestions for the troubleshooting section. I have stated what I'd like to prove. I have laid out how I want to proceed. This is just another test. Can anyone please guide me quickly and simply in what I intend to do?

But even so, most tube amps do need a certain amount of volume before they get in their sweet spot.

Thank you, JoeM!

I want to see if the spurious artifacts go away when I conduct this test. It already goes away when all four 6L6s are shattering my eardrums. I want to conduct this test at lower volumes, hence the wish to install a bandmaster OT. Can I do this swap in order to drive two 6L6s instead of four without having to change other components in the amp?

[Tom Phillips]

Yes you can if you pull two of the 6L6s as you already proposed and change the 47 Ohm resistor in the feedback network to 100 Ohms since the BM OT is 4 Ohm output impedane. Reference the schematic at http://webphix.com/schematic heaven/...b763_schem.pdf
Note that the resistor change isn't absolutly essential ( i.e the amp won't be damaged it you don't change the resistor value) but it is a consideration to keep the feedback level the same as it currently is with your existing 8 Ohm output impedance OT.

Your B+ supply will be stiffer but you can experiemnt with adding some resistance if you'd like.

[deci belle]

Oh, Tom! Thank you for those points and the go-ahead. Yes, I have already experimented a bit with the NFB ground resistor and have a 100Ω resistor installed. I didn't notice any difference myself. The 4Ω Showman's 100Ω R is noted and thank you for that link too

I will go ahead and do the test— thank you all for your help!

[deci belle]

Just wanted to add that changing the value of the bias-pot resistor will be necessary.

[Tom Phillips]

Just wanted to add that changing the value of the bias-pot resistor will be necessary.

Try adjusting the bias first. You may not need to change the 27k to 15k. As long as you can set the bias to the proper value for the tubes you end up using the absolute value of that resistor (within reasonable bounds) does not matter.

[deci belle]

+1!!