Zenith phonograph amp to guitar amp

[Gerard]

Hi , I have some questions on a Zenith phonograph to guitar amp conversion. First I have to say is, I know some things about tube amps. I did sucessfully build a 5e3 kit and , I am now "hooked" on tubes. So I found this Zenith tube phonograph amp and wanted to convert it. It has 2-6BQ5 (el84), 1-5Y3GT and 2-12AX7 tubes. I pluged it into a single celestion vintage G12-65 cabinet and connected a phono jack to one of the inputs. Pluged my guitar in and had great clean sound coming out.It will not distort even when turned all the way up. The bass, middle and treble controls all seemed to work. I was impressed, and when my blues driver pedal was put in front ,this thing screamed. Now for the questions. Do I have to do anything circuit wise to use it for a guitar amp? Like change the tone cap values? Can I take the PT, OT and tubes out and use a 5e3 circuit inbetween for the tone and volume? Should I just recap it and leave it alone?
Also too it seem's to be running the output tubes hot 360v on pin 7 (the plate) (300v max), even with the bias pot all the way down. Do I need to add more resistence to the grid(pin3), or decrease resistence? Or could I use a 7189 tube with a plate rating of 400v. Thanks in advance for all your help!
Gerard

[olddawg]

Sounds like a great chassis to easily build a Marshall 18 watt clone from to me. Everything is there. Where did you find this chassis and what model is it out of? Great score!

[Gerard]

I found it locally at a "sell it on ebay" store for $12.99, It was just the chassis, no tubes. The chassis is ink marked (chassis 5b23) and metal stamped 6578099, The OT is 95-1604 and the PT is 95-1603. That's all the markings on it. Thanks, Gerard
.

[Chuck H]

Do I have to do anything circuit wise to use it for a guitar amp? Like change the tone cap values?

No, you don't have to change anything if you like the way it sounds now. Thats the obvious answer. But it does sound like the amp is lacking in gain for a guitar amp if you can turn it up all the way and it won't distort at all.

Can I take the PT, OT and tubes out and use a 5e3 circuit inbetween for the tone and volume? Should I just recap it and leave it alone?

Same answer. Thats up to you. After you get it biased up properly, put a clean boost in front of it to see if you may like it stock but with more gain. If you like it increasing gain is usually pretty easy to do. You would have to learn to do it yourself or trace the circuit and post it here for advice. If you prefere the 5e3 circuit you could use the parts to build yourself another. Or since you already have one of those, someone else suggested an 18 watt clone. Also very cool.

Also too it seem's to be running the output tubes hot 360v on pin 7 (the plate)

Once you bias it up properly the plate voltage may drop a little. And despite the specs you can run el84's at 360 without problems. I've even seen them over 400 volts and 380 on the screens and survive.

(300v max), even with the bias pot all the way down.

Cooling the bias will actually increase plate voltage. you'll need to bias the amp properly to know what the plate volts really are.

Do I need to add more resistence to the grid(pin3), or decrease resistence? .

This would be an absolutely incorrect method for trying to change an amps bias. You should do some research online. There are many sites with info on how to bias a tube amp. Someone here can surely help if you still have questions. But there is literally gobs of biasing info already out there so it's silly to cover it a step at a time on a forum like this. I will say this much, you probably want to be drawing about 30 or 35mA per tube at idle. If you know what this means this should be all the info you need. If you don't then you need to find out before you can proceed anyway. Learning how to bias the amp will teach you more about how the power amp is operating too and help you to avoid mistakes like increasing the grid resistors in an effort to bias the amp or turning the bias adjust down to decrease plate volts.

Or could I use a 7189 tube with a plate rating of 400v. Thanks in advance for all your help!
Gerard

I would only trouble with 7198 or el84m tubes if I had plate volts over 400 or no good way to drop high screen volts. You should be fine with regular el84's. I'll bet that you'll be under 350 plate volts once the amp is biased properly.

Chuck

[Gerard]

Wow, Thanks for all the advice Chuck!, I am from the school "if you like it the way it is leave it alone" I will recap it though. It is lacking in gain and would like to attempt to change that. Is that done with changing the resistence to the pre amp tubes? This is funny the way it is wired, the input from the guitar goes directly to the volume pot ? unlike the 5e3 I built. I am kind of lost with that. Any thoughts? Thanks, Gerard

[Chuck H]

I've seen alot of old console amps wired input to volume control. This isn't a good thing for guitar. Changing the input circuit and a couple of other things (like new filter caps) may be all that is needed.

You can draw a schematic by tracing the circuit from the tube pins out. It will look pretty strange, and it's hard work. But that way we can offer advice on what you may want to change. Most phono, console and organ amps require very few changes to make them good guitar amps. Usually they are based on example circuits published by tube MFG's, as were mose early instrument amps.

Chuck

[Gerard]

I was wondering if I could rewire the whole thing using some of the tube layouts found on the web for a Marshall 18W. Just keeping the PT. ,OT and all the tubes I noticed that the 18W marshall has 3-12ax7"s, Do you think that would pose a problem with the PT output voltage?

[Chuck H]

Not at all. The current from adding a 12ax7 is mice nuts. I've added a preamp tube to old phono amps with no trouble. The filament winding should be over designed by at least that much. You could also build an 18 watter with just two preamp tubes and it would be just the same only one channel instead of two.

Chuck

[Gerard]

Thanks for all your comments Chuck! I will have to take a close look at that amp and see what I want to do.

[bluesbreaker18]

Converted Zenith phonograph to 18watt

Gerard,

I recently faced a similar dilemma. I created an 18watt lite and I'm very happy with it. See http://music-electronics-forum.com/s...ad.php?t=12577 for more information and links to pics.

Good luck!

[Gerard]

awsome job! It's great to recycle and create.

[petemoore]

RCA Stereo w/EL84's was so sad, it sounded superb before the PS needed rebuilt..
And the whole front end needed work, and the tube sockets got slacky.
Fortunately other services required the output irons, and 1 GDS/Heyboer OT was placed where they were.
The preparations for the molting and Phoenixing process began, with the BOM...
And a pull of the 2pronged deathwire, install of IEC and Green GND wire eyelet boltdown to chassis, test to GND at plug end...and sleeve of input...which I insulated from chassis, has GND wire.
Hasn't really ended yet...I still have more sockets I think should be filled, as it is the third tube stage really isn't needed [18w Lite], and it could use more TC, maybe a reverb. This amp has room in it still for more tubes.
A fair part of the initial slow thinking involved laying out the 18watt to the squarish shaped chassis of probably similar 'volume.
All new caps are so nice next to old transformers and resistors [value measured of course].

[chihuahua1969]

hi, I have the same exact amp. where is the on-off switch...I found the chassis with tubes, but no indications of the knobs, and maybe one of the socket terminals turns the thing on??I juts don't know..thanks, Chris

[olddawg]

If you are going to convert to an 18 watt Marshall clone or variant the drill is always the same. Gut the chassis and start over. Go to 18watt.com and chose a known good and debugged schematic and layout. (Brown Note, etc) Replace all of the tube sockets with new ceramics. If you want more heater current, use a solid state rectifier and an LED pilot lamp. Build up a new turret/eyelet board from new fresh components. Use new quality pots and jacks. You will find that all of this will not be expensive at all and will save you hours of debugging and frustration. Also, these are not good amps to add a reverb or an effects loop to. You will get much better results placing all of your effects on the input. Another good amp possibility is a Vox AC 15. I gig clubs with an 18 watt Marshall variant that has some Zenith parts. I mounted the chassis in an old Lead 100 MOSFET head box and use the matching red 1965A cab. People are always asking me what the hell it is because it looks sharp, pro, and I can dime it in a club without a power soak. Most think I'm BSing them when I say I built it out of old Zenith parts.

[RWood]

hi, I have the same exact amp. where is the on-off switch...I found the chassis with tubes, but no indications of the knobs, and maybe one of the socket terminals turns the thing on??I juts don't know..thanks, Chris

In the console originally there was a tuner/preamp that had the system's power switch on a push-pull volume pot. You can follow the power cord coming into your chassis and see which socket pin it is going to, then returning from. But even if you do you're going to wind up with a power amp.

If you are here because you want to convert it into a guitar amp, follow Olddawg's advice and use what you've got - the expensive parts - to build yourself a guitar amp like the 18 watt.