Quick And Dirty ~75-watter

[sorenj07]

Hi, just asking the feasibility of the following amp. A fellow senior at my school just finished building a PIMPING guitar (maybe he'll send me a pic or two) and would like a pretty solid tube amp to accompany it. His preliminary idea was around 75 watts, with some basic controls such as overdrive, bass and treble. Here are my thoughts. I'm an amateur at guitar amps, so let me know if something is absolutely infeasible.

Output tubes - 2 KT88's. At guitar frequencies, 75 watts should be possible, right? I might use a single SS diode to tap off a negative bias.

Driving tubes - 2 12AX7's, 1 12AT7. The 12AX7's serve to jack up the gain, and at least one triode will feed a filter network. I have Duncan Amps' Tonestack Calculator and TubeCAD, and I have basic biasing knowledge. Each half of the 12AT7 will drive a KT88. The OPT would be one of Edcor's 5K 100W deals.

PSU: I'm thinking a 282X (200mA 1000VCT) feeding a 5U4, into a small filter cap, then a 193H choke (5H 200mA 65R) and a decent filter cap. RC's to feed the 12AT7 and 12AX7.

This is a pretty sketchy layout but does it seem feasible? His budget is around $600. Nothing amazing for the tubes, probably Sovtek. Does anyone happen to have a schematic lying around that looks like it could work? Thanks!

[SteveH]

Hrmmm....Kinda funny you make this post.

I've been looking around for something similar functionality wise; but I am looking in the neighborhood of 40-50w. Thinking of going with EL34's if I can squeeze enough power out of 'em.

[EWBrown]

A pair of KT88s or 6550Cs should give you 75 Watts, just look at the "Eiclone" project for a good idea of how this happens. That uses a 6SN7 for a phase splitter, but a 6CG7 or 12AT7 could be worked into the design, fairly easily.

You might find some inspiration / ideas over at Eurotubes.

1000VCT may be a bit high, the Ike can deliver around 75W with an 800VDC / 200 mA Hammond PT (or use the Triode P-782 power trannie, it has "gorilla balls" power handling capability) Yellow_Light_Colorz_PDT_06

If you go with tube rect, maybe 450-0-450, maximum.

/ed B in NH

[dhuebert]

Have a look at the Hammertone Sixpac, with a few mods it can give 75 watts.

Replace the 6L6s with KT88s or EL34s, bump B+ to 475V or so by changing the 276X to a 274BX and replace the output transformer with the following:


TRANSFORMER, FENDER ORIGINAL, OUTPUT, HOT ROD DEVILLE

P-TF41754

Replacement for Hot Rod DeVille. 2/4/8 ohms. Equivalent to #041754.

Specifications
Wattage : 60
Primary Impedance (Ohms): 4200 C.T.
Secondary Impedance (Ohms): 2/4/8
Mounting Centers : 2" x 3" vertical
Weight : 4.7 lbs

http://www.tubesandmore.com/

Make sure your power supply caps have the correct voltage rating and replace the 1N4007s with 1N5408s ala the hammertone BFA. Tube rectifiers are BS, there I said it!

On second thought, you might want to go with the EL34 or 6CA7 based purely on availability. Any guitar shop can supply them but if a KT88 went out you will have to wait for a new one.

I like 6L6 and a quartet will do what you want nicely. The Hammertone Viper burnt a 6L6 while on tour and the owner pulled one out of a friends amp to finish the tour. With this in mind, I would suggest 12AX7, 12AT7 and four 6L6.


Don

[EWBrown]

JJ, and Elektroharmonix, and other Russian made and re-branded KT88s are pretty readily available. The "real" British-made Genalex, Gold Lion, NOS, now that's another story altogether :o :o :o


Ditto for 6550Cs. Not all EL34s are created equal, some are OK for 35-40WPC stereo use, but start thrashing 'em like a rented mule (guitar amp service) and they may go pyrotechnic. Aviod the "Sino" Chinese made EL34s, some of them can be rather whimpy Yellow_Light_Colorz_PDT_09

OTOH, 6L6GCs are nearly indesctructible (short of dropping them on a cement floor) and they are cheap and can be found just about anywhere.

/ed B in NH

[dhuebert]

One of the modifications Garnet made to Marshalls to make the EL34s last longer was to hook the suppressor grid, G3, to the bias supply. He claimed this extended tube life dramatically. It only works for tubes with G3 attatched to pin 1, tho

Don

[sorenj07]

Have a look at the Hammertone Sixpac, with a few mods it can give 75 watts.

I will, and I really appreciate the careful schematic work you did. I'm not a pro at using turret boards but it should be OK to figure out.

Replace the 6L6s with KT88s or EL34s, bump B+ to 475V or so by changing the 276X to a 274BX and replace the output transformer with the following:

TRANSFORMER, FENDER ORIGINAL, OUTPUT, HOT ROD DEVILLE

P-TF41754

I would, but couldn't I just use one of those 4.2K-8R 60W Edcors instead? They're rated down to 20Hz so I think that 75W starting at ~50Hz or so isn't too much to ask..

Tube rectifiers are BS, there I said it!

I agree with you here. This dude wants a really aggressive, punchy amp for hard rock/metal. Thanks so much for all the advice! I'll get to work on a schematic so we can start ordering parts.

One last question. I assume that for guitar amp use, I ought to use a mono 1/4" jack?

[EWBrown]

Actually, in the case of a guitar amp, a less than "golden-ear audiophile" quality trannie is a good thing, and the Edcors should work just fine. I don't know if there is anything "magical" about the Fender OPT's winding / construction. Triode has some 4.2KP-P EL84 guitar amp OPTs, but their rating is "only" 30 watts (50-20KHz). Though, with its 100 mA current handling capacity, it could probably pushed harder, though 75 watts may be pushing just a tad TOO hard...

The 60 Watt Carver OPT I have is very small, as compared to an equivalent impedance / power handling Hammond audio output trannie, it is 6600 ohms CT, intended for 6L6GC service, fixed bias.

Even the relatively small $23 Triode TF-110-48 UL can be pushed to 50 watts in guitar amp service, though it's more comfortable at 15-20 WPC in "hi-fi" usage. The paper bobbin version has similar ratings, though it is physically somewhat larger, and costs $17 more.

1/4 inch mono phone jack is the default standard guitar amp input connector .

As far as what is the best speaker to use for hard rock / metal, you might want to refer to the Parts Express catalog, or Antique Electronic Supply, to figure out what would be the best match. Eminence has a wide variety and Celestion or Jensen are also very good.

/ed B in NH

[nyazzip]

as far as speakers, i recommend the celeston g12t-75.....the current fad is to be terrified of them, and though they were recently worth $115 new, you can currently get them on ebay for 20 bucks, because as everyone knows, alnico is the only way... till next year.
the g12ts exhibit a pretty ragged freq curve, with lots of midrange spikes, perfect for hard rock/metal, so the literature goes. i had one, just sold it. sounded fine, a bit trebley for me but i use strats and "natural" distortion with open cabinets
95% of a nu-metal guitarist's sound will come from the array of digital effects in line before the signal ever reaches the amp-
amp topology/tubes/wattage/speakers, these considerations are moot when metal is the music being played, because they just don't tend to factor in heavily
buy the kid a Line 6 with modeling and DSP and call it a day

[dhuebert]

I assume that for guitar amp use, I ought to use a mono 1/4" jack?

JACK, MONO INPUT W/ FERRULE, 4 SOLDER TERMINALS, SWITCHED

S-H601

Mono input. Solder lugs, shorting. Metal ferrule, vintage style. Similar to S-H501.

Please click on image for further spec

RoHS Compliant

From Tubes and More.

Ground everything up to the phase invertor directly to the jack and ground the jack to the power ground where it bolts to the chassis.

Having said all that:

buy the kid a Line 6 with modeling and DSP and call it a day

Everytime I hear a Fender DeVille I wonder why I waste my time building guitar amps. I've seen half a dozen pedals into an amp and an amazing array of sounds come out. As for building; do it for your own amusement and don't worry about making money, that's for sure! The Hammertone Champ is a good example of that, I built it cause I wanted to, it just so happened a friend liked it so it has a home.

Don