how to get more out of a Vibro Champ

[Groover]

I play some recurring church and theater gigs where space is a premium and getting a good sound at a very low volume is important. I have used various amps - My stock Blues Junior is waaay too loud, I barely crack the master. My Tweed Harvard doesn't break a sweat. My Vibro-Champ with Weber 8F150 is the closest, but gets flabby in the bass register.

I would like to be able to goose the Vibro-Champ enough so that the low end would stay solid, but still keep the power low enough that it still sounds fat.

FYI The Vibro-Champ has a Mercury PT already, which can handle 100ma according to Mercury info.

Sooo there are a couple of ways to go - either stay with the single ended power section and try by using a bigger OT to get more low frequency headroom, maybe going to a 6L6 - or - replacing the SE power amp with a low power push-pull output section. (I'm thinking cathode biased.)

I turn to the collective experience of this forum to shed light on
a) how much more ooomph a bigger OT would give me in a SE Champ
b) how one could limit the power output of a pair of 6V6s to around 10 watts.
c) which course of action might work out better for me.

[txstrat]

A bigger SE OT should give you a little more ooomph (depending on the size of the OT and the value of the cathode resistor).
I think converting to PP would leave you closer to your Blues Junior headroom wise. Even when you choose a small OT and bias cold.
Easiest way could be to change the power (6L6) and/or rectifier tubes so the amp behaves stiffer at higher voltages.

Just my 2C

[MWJB]

If using a 6L6 in a champ upgrade the cathode bypass cap to 100v rating, the cathode resistor should be 10W. Betters still go fixed bias.

The stock PT will be happy enough with a 6L6, but increased heater & B+ current draw will pull the plate voltage down to under 400vdc. Fixed bias with a 6V6 might see around 450vdc on the plate (use EH/RI Tung Sol/JJ)? Your Mercury should do a bit better voltage wise, especially if the heater winding is upgraded to more than 2A (3A would allow 6550/KT88, add a 1K 5W screen grid resistor). I'd recommend setting up to allow both 6L6 & 6V6 operation and let your ears decide, the see-saw effect with the plate voltage might be more relevant than type of power tube.

First stop would be to try GZ34/5AR4 or a solid state rectifier, with a bigger OT. 100uf for the main B+ filter cap, 100uf/100v for the power tube cathode bypass cap if sticking with cathode bias. A 680-800ohm cathode resistor should be a good compromise for 6V6 & 6L6. 10" speaker.

As Txstrat says, going push pull is going to put the amp pretty well on par with the Harvard?

[Groover]

Opinions on the following OTs?

Weber SE 15 - 15watts, current rating not specified, $35
Hammond PT125E SE - 15watts, 80ma, $56
Allen Amps TO11C - wattage not specified, 95ma, $39

They will all probably require creativity in mounting due to increased size.

[rocket]

Isn't a 10" or 12" speaker what you really need?
You might try the other amps' as extension cabinets and find out which one is best.

[Groover]

Yes, but with the current OT the impedance mismatch will be pretty large (I already have a 6L6 in the amp so I think it really wants to see somewhat less than the stock 3.2 ohms) - I don't know if it will really be that informative into a 8 ohm speaker. Now I do have a 4ohm EV15" available. I'll try that.

[rocket]

I have a small DIY 5W SE combo with a 6V6 and a Celestion Vintage 10.
There is no lack of bass and neither is with the 12" Weber California, I use in an extra cabinet.

I think a bigger speaker will much more likely cure your problem than a higher power tube.

[Groover]

Rocket, what kind of OT did you use in your SE combo?

FYI I am probably going to go to a 10" speaker once I get a new baffle board and an OT with an 8ohm tap.

[J M Fahey]

I'd touch nothing *electronic*, but build a slightly larger cabinet , we are talking a few inches here, and mount in it the champ chassis and a somewhat larger speaker.
If you are happy with your Weber, there exist the same magnet and voice coil but 10" or 12" versions.
Any mod addressing the electronics part will make you *louder* (I don't think you want that) with about the same sound (which you don't want either).
Just my 2 cents.
EDIT: if your transformer does not have an 8 ohm tap, a 4 ohm speaker into a 3.2 ohm tap is the same, in practical terms, specially talking about SE amps.

[Groover]

I tried the amp into my 4 ohm EV 15" speaker and it is bigger sounding with more lows, but was not as dramatically different as I expected it to be.

[J M Fahey]

Thanks.
It's always good to know what happens in the Real World, compared to what theory would imply.
I've always found EVs and other heavy PA type speakers impressive and chest thumping when driven with big watts, but many times also found much lighter , guitar specific speakers provide excellent, loud sound.

[rocket]

[QUOTE=Groover;144510]Rocket, what kind of OT did you use in your SE combo?

The OT is from a German supplier (Frag Jan zuerst --- Ask Jan First), the AT288.
It is quite massive with a EI84 core.