Amp Tech Thread / Ask a tech Q

[Hemisaurus]

Would that be a multi-channel amp?

In tube-land, gain is rarely actual gain, it's generally a volume control before a fixed gain stage, so that you can overdrive, or not overdrive the gain stage with the gain knob and then turn the thing down clean or dirty with the volume knob.

Input > Pink Channel > attenuation > gain stage > attenuation > mixer stage > master volume > power amp section
                                      Gain                         Channel Vol


If you have a specific unit in mind, you can check out the schematic, or I can

[The Shocker]

It's a Fender Mustang I amp - been looking for a small practice amp & it looks like fun.  It's a single channel amp, but footswitchable between 2 amp model presets (?).

Still gain, volume and master volume has me confused on this one...

[Hemisaurus]

Looks like it's like my VAMP, and like PODs and other modelling doofers.

The Gain/Vol/Bass/Treble are under software control, probably rotary encoders, they are feeding parameters to the model. The master volume is an actual pot and controls the level of the modelled signal.

In theory, this allows you to dime all you modelling controls so the modelled amp is running full on, with maximum power tube crunchiness, and then you can turn it down with the master volume, as it's a physical control.

Make sense?

[The Shocker]

Yeah, I guess.  I mean I understand Master volume & gain in tube amps, but with a solid state modeling amp it seems kind of wonky.   

[Hemisaurus]

It's way easier to get gain on a solid state amp

If you imagine the modeled amp as being a small amp in a big box with a window, which is the amp modeller. The small amp has 4 knobs on it, like many amps. Then there's a knob on the big box to control how far you open the window.

[The Shocker]

Great explanation, thanks!

[inductorguitars]

<crickets>

[justinhedrick]

<crickets>

inside your amp??!! get them out!

[Hemisaurus]

[VOLVO)))]

HAHAHA TOM'S AMPS!

I'm going to message him on facebook.

[Hemisaurus]

Herbie should post the "how to bias" thread now... I'm gonna have to retube it now... engh... son of a bitch.

Any idiot can bias a Concert Lead

I say, as per manufacturer, or failing that, as per the equipment you have to hand.

Does your amp have a bias point, do you have a bias probe, do you have a scope?

http://tone-lizard.com/Biasing.htm

if there's no manufacturers bias procedure, and there is an adjustable bias, I'll use a scope.

[Hemisaurus]

https://ssl.eurotubes.com/cart/index.php?page=view_products&category_id=108&sub_category_id=109

or

https://taweber.powweb.com/store/modkits.htm#BR-Kit

or just make yer own.

[Slow]

Swank tits, is it worth the investment? I'll buy my tubes from there, too. I'm kind of sad to see these tubes go

[inductorguitars]

The stock JCM800 is fixed bias.. isn't it?

[Slow]

bullllllsshit...

[Hemisaurus]

PS, Sunn can you cut this end of the thread, and paste it into tech thread?

[VOLVO)))]

indeed! so the 800 is fixed bias? that would give me a woody.

[VOLVO)))]

oi, thats the bias pot. derp. I can read schematics, I just don't know how to apply it to the physical realm... R35, R36, R37, R38 are my screen resistors?

What you're pointing to is a trimmer, or a pot?

Which leads me to my next question, Do I have to bias each socket, or is that a master, so I can just read one socket and set from there?

[Hemisaurus]

If you are buying a bias probe from eurotubes, you might as well follow his instructions too. I solemnly believe there is more than one way to skin a cat, or bias an amp in this case.

http://eurotubes.com/euro-Generic-Bias.htm

He just happens to use a JCM 800 as his generic amp

It is a trimmer or at least it's a non-front panel adjustment, depends on your model where it is, you got one of those whacky PCB's so I don't know where it will be for you. It would be fairer to call it more of a bias balance as it affects all the tubes at once, you have to check each tube, and use the adjustment to get a happy medium.

[VOLVO)))]

Solid, so it doesn't have to be exact?

I understand I can use 6550s in it, but someone told me I had to add something, or change some resistor values somewhere. T/F?

[Hemisaurus]

T.

I think you could drop KT-77'S in w/out mods.

[VOLVO)))]

Sold. That's what I was going to buy. This is exciting...

[bass sic]

A quick question for Hemi or anyone with 4pro experience. Right now I'm running a 1/4 plug from channel A or B output, and was reading that if you have the stereo/mono button in the wrong position you won't get full power. I also read some shit about running mono bridged mode using the speakon connector. Well I got the speakon plug but can't set this thing up till Saturdays show. Is it as easy as a plug from mono/bridged output into 8olm cab? The rear panel on this thing is kinda busy, with a shit ton of config options.

[Hemisaurus]

Manual http://www.ampeg.com/pdf/SVT4PRO.pdf

Page 12 - Note that a speakon cable for the A/B output is not wired the same as a speakon cable for the Mono out. If by plug you mean you bought just the plug, and plan to make your own cable, that's OK. If by plug you mean the whole cable, you may need to double check it.

Also if the Speakon is an NL-2 and not an NL-4, it does not have the required terminals for the Mono Output, which is wired between 1+ and 2+ instead of the standard 1+ 1-, this is the 'bridge mode' standard for most amps.

Quick Table for Speakons

Regular Cable +ve=1+ -ve=1-
Biamp Cable Lo +ve =1+ Lo -ve=1-
                    Hi +ve =2+  Hi -ve=2-
Bridge Cable +ve=1+ -ve=2+

[bass sic]

Why you wanna fuck me all up with equations and shit. What I got was a speakon to 1/4 plug. Now I'll have to see if it's for mono mode. I've read thru that manual a few times, but a lot of it is foreign to me.