Is a 6G6-A Bassman quiet for it's power?

[pokejohan]

I've been lurking on Ampage boards since around Y2K, but have never posted - so first of all, thanks for hours of great amp tech reading.

Second, do any of you have a real-deal Blonde Bassman, and if so does it seem... well... "quiet" for its power?

I built a clone of a 6G6-A Bassman, and while it sounds incredible (best sounding Amp I've built by far), it seems pretty quiet for what's supposed to be a 40-50W amp - that's compared to my 18W Marshall clone - the little EL-84s can definitely keep up with the Bassman's 6L6s as far as percieved volume goes.

Measured across a 1 Ohm precision resistor between ground and the pair of 6L6 cathodes, I typically get 33ma idle current.

When set to about 7 in the normal channel, an open E will push it to around 120 ma... assuming a negligible amount of screen current (measurements bear this out) that measures out to be approximately

Ia*Va =
480V * .120A =
57W

So.... My math tells me that everything is copacetic. My ears tell me it should be louder - but then, I've never heard the original.

BTW, we're playing through a pair of 12" Eminence Legends (can't remember the exact model) through a closed back cab built to the dimensions of the original Blonde Bassman cab.

Thanks for any input.
Jonathan

[gldtp99]

I recently built a head using the 6G6-A clean channel as a starting point for the preamp---- it's been through many revisions and is now closer to the 6G6-B clean channel ----- i'm running Marshall-type 50 watt iron and EL34's and it has the punch of a 50 watt amp.
What are the Voltage and Current ratings of the PT you're using ? What kind of OT are you using ?
A PT without enough current capacity can't push the pair of fixed bias 6L6GC's to their potential max audio output---- It would get to a certain vol output and no louder---- also an underrated OT could also be a bottleneck as far as Vol/Tone goes.
Post internal pics of the amp if you can...................gldtp99

[tboy]

I've had a few of those, and I can assure you they're not at all quiet.

You're measuring the combined current of both power tubes through that common one-ohm resistor. That's only 16.5 milliamperes per tube, or about eight watts if you have 480 volts on the plate, which is pretty cold compared to the often-recommended 70% max dissipation at idle.

Output power and plate dissipation are two different things. Output power is measured at the speaker leads.

[Dave Curtis, dB AudioTech]

Like one of these?
http://www.flickr.com/photos/8993818...7600381417325/

Pretty quiet once I was done. BTW, he also has the matching cab & covers.

[pokejohan]

Thanks folks.

gldtp99:

The OT is a New Sensor NSC022814 - supposed to supply 140ma at 690VAC PtoP - so that shouldn't be a problem. OT is a new sensor "fender replacement" 40W OT, NSC022871. No tell-tale excessive heating of either the OT nor the PT after playing through it for 30-45 minutes.

Myself, I've been partial to Hammond xformers on my homebrews, but this build was for my dad, and he wanted to get iron that was labeled "replacement transformers" for the particular amp in question.

tboy:

Yeah, I agree, that's a pretty low plate dissipation per tube. I was able to max out the bias voltage at -45v (I think it was) and got the idle current through both tubes up to about 50ma, but the 33ma I mentioned was at the schematic reccomended -58. Since my plate voltage ended up being pretty much spot on and it sounds great, I thought I'd leave it at the reccomended bias voltage - at least till I figured out if I had a problem. In any case, I doubt you should have the bias pot at one extreme or the other.

My musing was that perhaps the amp was designed for a cold bias - not likely for a bass amp, but then by all accounts it was never a particularly good bass amp anyway.

When I've spoken of an amp's power, my verbage has always been based on plate dissipation - Never really considered checking at the speaker leads.... Not real sure how I'd go about checking that.... signal generator and diming the volume knob? Measure the voltage and go (V^2)/(impedance)?

As far as pictures go, I'm not particularly proud of the way this amp looks on the inside. We ended up using a chassis that was a bit too narrow so I'm a bit cramped inside. We plan on rebuilding into a stock sized chassis later in the year and I'll post amp porn then .

Thanks, Jonahtan

[pokejohan]

whoops... forgot to mention in all of this that I added a bias adjustment pot. I'd forgotten that they didn't come that way stock.

Jonathan

[pokejohan]

Sweet! Nice pics and nice job. The blonde and brown fender amps have always been my favorites cosmetically. I don't suppose you measured any idle current readings through the power tubes or off the plates did you?

Jonathan

[MWJB]

33mA per tube at 480v would be spot on (e.g. 66mA at a single 1ohm cathode resistor shared by both power tubes, or 33mA at each pin 8 if you have 1x 1ohm resistor per tube).

To measure output connect a true RMS AC voltmeter accross the speaker terminals and play, reading will fly about somewhat but try and determine an average voltage. Square that voltage, then divide by speaker load in ohms. You might be getting about 13-15VAC?

As Tboy states , Watts AC output power & Watts dc plate dissipation are different perameters and are not directly related.

Comparing a 2xEL84 amp and a 2x6L6 amp at home can be misleading. A cathode biased 18W will be touch sensitive and appear loud from the off, the fixed bias 6L6 amp may need to be at a much higher volume to get into it's stride but put drums & bass behind them and the 6L6 amp should hold together much better. Differences in voicing could actually make an 18W appear louder, especially with no background.

The bassman only makes 2 1/2 times the power of the 18W, not a great deal when considering "perceived volume".

[Steve Conner]

120mA of cathode current seems a little stingy for full power. The amps I've built can get up to about 300mA when they're cranked, with 6L6s or EL34s.

Also, the 18 watter is loud for its power. It overdrives its power tubes a lot, and the resulting grind and distortion make it seem louder. A Bassman would be cleaner.

All I can suggest is to try a clean boost pedal on the Bassman, crank all the knobs up full, and see if it's still too quiet for you.

[pokejohan]

MWJB:

Thanks for the input and the tek on measuring the power at the terms. I'll do that next time we're playing with the amp.

Right now it's 33ma through both tubes, so I may adjust the bias to run it a little hotter.... but I'm not going to worry about it too much since it sounds killer. When we rebuild the amp this summer we may find that I have some cold solder joint somewhere doing freaky things. Maybe not.

Good point on the fixed vs cathode biased power amps. If that is indeed a big part of the phenomenon we're experiencing here, it'll make me lean toward cathode biasing my future amps - of course then it wouldn't sound like a blonde/brown/blackface fender anymore I guess... hmmm ... BF preamp and an 18W poweramp... hmmm (I smell a future project).

Steve:

I'm not sure what the cathode current would be with it dimed... I think that the 120ma reading was with the normal channel turned to around 7 on a blackface style knob. And I really don't mind the fact that it's quieter than expected myself - it has a beautiful chimey clean sound, makes a strat sound like a strat. ... but since I built it for my dad, I kinda feel a responsibility to make sure we're getting "what he paid for" out of the transformers, if you know what I mean.

[MWJB]

."Right now it's 33ma through both tubes... but I'm not going to worry about it too much since it sounds killer." I would expect the tone to be thin & scratchy at 16.5mA per tube, i would want to be in the high 20's, at least, if I wanted the amp to sound typically Fendery.

It's not just the difference between cathode & fixed bias, the tubes make adifference too. EL84 are particularly efficient in cathode bias, compared to 6L6. Easy enough to add a fixed/cathode to your bassman, see Londonpower.com & trawl through the FAQ page.

[pokejohan]

I would expect the tone to be thin & scratchy at 16.5mA per tube, i would want to be in the high 20's, at least, if I wanted the amp to sound typically Fendery.

Ever heard John Hiatt's album Slow Turning? There's a song on there called Icy Blue Heart. Put my Strat in the bridge position playing through this amp and it sounds pretty much just like the album.

Does a good job of sounding like The Beatles' "I've Got A Feeling" opening riff when you play a humbucker in the neck position.

I'll try to get a decent recording of it this weekend when I get back down to my dad's.

[Dave Curtis, dB AudioTech]

Sweet! Nice pics and nice job. The blonde and brown fender amps have always been my favorites cosmetically. I don't suppose you measured any idle current readings through the power tubes or off the plates did you?

Jonathan

Yes, of course, but I'd have to dig through the repair notes and see if I wrote it down... I'll get back to you.