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View Full Version : Guitar vs stereo amps


MVrider
03-21-2012, 08:45 AM
There are lots of guitar amps with 12AX7 and and four EL84's. There are also stereo amps with pretty much the same tube complement and power output (granted, in two separate output stages through two output transformers and stereo amps require a separate preamp) but generally speaking, what makes a stereo amp different from a guitar amp with a similar layout?

'58Bassman
03-21-2012, 09:35 AM
There are lots of guitar amps with 12AX7 and and four EL84's. There are also stereo amps with pretty much the same tube complement and power output (granted, in two separate output stages through two output transformers and stereo amps require a separate preamp) but generally speaking, what makes a stereo amp different from a guitar amp with a similar layout?

A stereo amp usually is designed to stay clean, so the gain is less, from each stage. Also, they tend to use more negative feedback, which removes specific types of distortion. This can be defeated and you may find that the gain increases. Also, a guitar amp doesn't need to provide a flat frequency response, like a stereo amp. In some cases, the first stage can benefit (for use as a guitar amp) from adding a cathode bypass capacitor-a lot more gain and you can change the low frequency response by using different cap values- larger lets more bass through and a smaller one raises the low frequency cutoff.

What kind of amps are you considering? People have modded stereo & mono amps to work for guitar- Bogen and Stromberg-Carlson are two brands that are fairly popular for this and they usually have a metal case and a handle, which makes them very portable.

WesKuhnley
03-21-2012, 10:01 AM
Take it from someone who's just finished doing a TON of research on the topic, the requirements of a hi-fi amp are FAR, FAR different than those of a guitar amp. Tube complement has nearly nothing to do with it. Powersupply, topology, gain structure, negative feedback etc are all different than in the guitar world.

donnyjaguar
03-21-2012, 11:40 AM
Musical Instrument amplifiers produce music.
Hi-Fi/stereo amplifiers reproduce music.

As noted above, the design approaches are indeed different. I always recommend people learn about tube hi-fi first. Just learning about MI amplifiers does not a good technician make, IMO.

Once of the differences betwixt Hi-Fi and MI power amplifiers is the quality of the output transformer. You can certainly use a hi-fi transformer for MI but the opposite is seldom true. :)

MVrider
03-21-2012, 05:24 PM
Thanks to all. Yeah. I remember reading somewhere that Ken Fischer liked to use Dynaco transformers in his Trainwrecks.

Keyser Soze
03-21-2012, 10:10 PM
Hi fi amps are designed to amplify with minimal alteration in sound.

Even the 'cleanest' instrument amp provides significant alteration of the input signal.

Plug your guitar into a hi fi tube amp and see what you have (not) been missing...

dspellman
03-21-2012, 10:16 PM
There are lots of guitar amps with 12AX7 and and four EL84's. There are also stereo amps with pretty much the same tube complement and power output (granted, in two separate output stages through two output transformers and stereo amps require a separate preamp) but generally speaking, what makes a stereo amp different from a guitar amp with a similar layout?

In terms of the power stage, usually the answer is "not much."
BTW, it's unclear if you're talking about hi-fi (like home audio) or just stereo guitar. There are certainly stereo guitar amps out there, from companies like Mesa, etc. One example is the Carvin TS-100, a stereo tube amplifier with 50W per side (or it can be bridged for 100W mono). Your choice of EL34 or 6L6 tubes (you can use a different tube for each side). Up to the rated power, THD is extremely low and response is good enough to work with any audiophile system.

MVrider
03-21-2012, 10:20 PM
Definitely talking stereo gear here. Might be able to pick up a homebrew EL84 stereo rig cheap and that got me wondering how tough it would be to mod it and use it for a guitar amp. For for the DIY aspect than saving any $$$$ So to clear this up yeah. Definitely talking turning a stereo audio rig into a guitar amp. Just wondering

diagrammatiks
03-21-2012, 10:20 PM
the circuits are in most cases completely different.

there might have been a point in the past where they were more closely related but not so much anymore.

that being said you could probably play guitar through a mcintosh 275 or a quad and it would sound great.

MVrider
03-21-2012, 10:39 PM
This is a little guy 4 EL84 amp, two per side so 15 maybe 18w per. Might be worth it just for the transformers and the chassis, couple other parts here and there.

donnyjaguar
03-22-2012, 09:57 AM
Some EL84 powered hi-fi amps can sound awesome. The lower-powered tube hi-fi can often produce remarkably sparkling highs. For a lower-powered system for office or sitting room they can be really nice.

smolder
03-22-2012, 10:48 AM
Garcia comes to mind reading this thread. His quest for more volume lead him to initially use Mac tube amps and he SS, while using a twin reverb on the front end (SRS out of St Louis, makes a preamp for this purpose). The macs clipping was kind of important to his tone at times, but he was largely looking for big, for clean, and for consistent. Many of the guys chasing his tone have tried other. SS amps with marginal results.

As a reference point;

http://www.wald-electronics.com/

It seams to me that using either solid state, or hifi gear is a decided shift towards getting your breakup (distortion) from either pedals, the preamp, or speakers. I completely missed the rack mount era of the eighties, so I've never built a rig this way.

guitarcapo
03-23-2012, 01:31 PM
Here's a famous "stereo" guitar amp: a late 1950's Magnatone 280B.

It uses two separate output transformers and four 6973 power tubes. (two pairs in push pull)

This isn't simply "two amps in one cab" Each power section reproduces a different pitch shifting vibrato signal so that when they combine in space between the speakers, you get all of these phase shift relationships that vary randomly.

There are other stereo guitar amps that actually amplify different signals from the guitar... but this one (designed in the late 1950's) was made to take a signal from the guitar and create a stereo modulating effect with it. Very amazing for the time it was designed.

The effect is best heard through stereo headphones from these demo clips. It was recorded with a stereo mic. Unlike digital stereo effects, this effect is more random between the two channels.

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Chrome Dinette
03-23-2012, 01:41 PM
Some Sunn amps are also very similar to hifi amps. In fact I think several have schematics nearly identical to Dynaco circuits, with an additional two gain stages at the front end. They also used Dynaco transformers, i believe.

wall_of_sleep
03-23-2012, 01:49 PM
This is a little guy 4 EL84 amp, two per side so 15 maybe 18w per. Might be worth it just for the transformers and the chassis, couple other parts here and there.
Reworking old gear is a favorite past time of mine. You can snag these guys on the bay from $40-60 every week. Aside from punching another preamp tube or tweaking the layout, most of these are near ready to go. Even the filter cans haven't given me any problems on the half dozen I've worked.

paulg
03-23-2012, 02:27 PM
I think a major difference is the voicing. A Hi-fidelity stereo amp will be full range. So, it will have larger value coupling caps between gain stages. A guitar amp, expecially, hi gain amps cut the low end. Guitar amps also tend to be based on primitive designs and very basic.