Replies to This Discussion

Permalink Reply by eric gittins on July 30, 2012 at 2:34pm

very cool fella.  do you have a schematic of what's there?  if not, i would try to trace through the wiring thats there to work out the connections to the transformers if nothing else.  this should tell you which conections are for the heaters, and which for the HT etc. if you can still read the markings on the valves, look up their pin-outs (google 'tube database'), and trace back to the trannies.  at least 2 of the valves you won't have to worry about (radio valves - usually have odd looking screens inside the glass). 

you are looking for the rectifier tube, power (amp) tube and a pre-amp tube.  the connections to these three tubes should tell you all you need to rewire it.

good luck!

Permalink Reply by Glenn Reither on July 30, 2012 at 2:34pm

I've done one, with a LOT of help from an electrical engineer buddy. 1) Be careful! There's a lot juice running around in there. 2) In my case, and I suspect with a lot of the valve radios, the tubes are pretty weak. We're going to mod it to turn one of the non-amp tubes into an extra gain stage.

I think some people are pretty much gutting them and putting a proper tube amp for guitar inside.

Permalink Reply by eric gittins on July 30, 2012 at 2:44pm

if the marking on the chassis is right, the EL90 is the power tube, and although it's a mini 7-pin, on paper it puts out very close to a 6v6 - about 4.5W.  You might have yourself the makings of a fine champ there.

tube data link here...

http://tdsl.duncanamps.com/show.php?des=EL90

Permalink Reply by GARAGE HERMIT on September 25, 2012 at 11:01am

been reading up on tube amp's and have come up with this circuit, it's probably wrong, cap,resistor value's etc, if someone can help me out and correct it i'd be very gratefull, 

Permalink Reply by eric gittins on September 25, 2012 at 3:31pm

looking good fella.

just had a quick look and seen a few things.

you won't need those two diodes from the power trans to the 6X4 - that's what the tube is anyway.  i would probably run a stand-by switch where you've drawn the diodes, but not vital with a tube rectifier.

your grid input of stage one needs a ref to earth to bias properly, so put a 1meg resister from the hot side of the jack to earth before the 47k res you've shown.

your 2k2 cathode resistors are pretty large, but that might be what you need to bias the 6AT6 - you could draw the load lines up off the data sheet and work out the sweet spot for the tube.  i would also bypass the cathode resistors.  caps could be anywhere from 1uf to 25uf (25v) - this cap will affect the bass response and can tidy up some ugly distortion when cranked.

i would recommend a grid stopper resister on the EL84. solder it right at the lug on the socket. maybe 5k6? this will help to make the distortion sweeter when driving the power tube hard. and can be required sometimes for stability.

i would also bypass the cathode resister on the EL84 with a 100uf cap (50-63v). the feedback from a cap here does reduce your gain, but will give you far superior tone IMHO.

other than that, it looks good.  measure all your voltages once you arc it up, and check everything is as it should be. i always like to calculate the actual current through the power tube too. just to make sure that's all good too.  max plate disipation for the EL84 is 12W.

i assume you are changing the power tube from the 7-pin EL90 in the original amp to an EL84 as in your schematic. the load for the output trans should be fine, but bear in mind that the EL84 uses a bit more heater current - 760mA vs 450 mA.  i would add up the heater current of all the original tubes just to see how it compares with your new load.

have fun, be safe, let us all know how it goes!

e

Permalink Reply by GARAGE HERMIT on September 26, 2012 at 5:32am

thank you very much for all that information, i'm still a newbie at all this tube stuff, so i'll have a few question's about the info in your reply,

but for now,

the ss diode's (1N4007) are insurance in case the rec/tube short's out, they protect the power transformer, so i've been told, they only cost penny's and only reduce the B+ voltage 0.6v, so it sound's good to me,

i'm using the small chassis from the radio and OT, 2- 6AT6 (in place of a 12AX7A), EL84 and a 6X4 rec, i bought a used PT that has all the output tapping's i need, 300-0-300, 2 x 6.3v heater supply's, so i'm ok there,

the circuit i've drawn is bit's and piece's from other circuit's, as i've not seen a circuit using 2- 6AT6's, so i'm unsure of the value's of the resistor's and cap's, what i cant figure out is how do you know when to use an electrolytic  cap or say a poly or ceramic cap in the circuit,?

when you say "bypass the cathode resistor on the EL84", what do you mean, put the 100uf cap in parallel with the resistor,? more question's to come, thank's,,

Permalink Reply by eric gittins on September 29, 2012 at 2:36pm

the diode thing sounds fine, although i've never seen a short here blow the trans.  the fuse on the mains side of the trans should blow anyway. and yes, the tube rect will drop way more voltage than the silicon blobs.

as to cap type - for some placements it depends on value/size to a large degree as well as purpose, different types of caps aren't available in all values. wilth the smaller value caps, there are multiple options, with potentially minor tonal variations. usually electrolytic (polarised) for power supply and cathode bypass caps.  600v rated poly-something caps for coupling.  i avoid ceramics personally, though lots a vintage stuff used them - if you stick in a single pot tone control you may need one.

yes, bypass the cathode resistor means to run the capacitor in parallel with it.  i had it ass-about in the previous post, and the cathode resistor actually creates neg. feedback, reducing the gain of the stage.  the bypass cap blocks AC and gives you back the gain the resistor took away. BUT this cap in conjuction with the internal resistance of the tube creates a local low-pass filter, ie. if the value of the cap is low, it will give you back your trebles, but not your bass.  anything above 10uf should give you everything for guitar, the fender standard 25uf is obviously fine.  you could use a .68uf if you were going for a bright channel perhaps.

to see what the bypass cap does, check out this calculator online.  your 6at6 fits in between a 12ax7 and a 12at7.

http://www.ampbooks.com/home/amplifier-calculators/cathode-capacitor/

e

Permalink Reply by GARAGE HERMIT on October 3, 2012 at 10:31am

this is the No 3 drawing, think i'm getting somewhere near for it to be ok, what do you think eric,?

the grid stopper resistor you mentioned for the EL84, would it go on pinout No9 or No2, ?

 

Permalink Reply by eric gittins on October 3, 2012 at 2:47pm

that's looking the goods to me. the grid stopper for the el84 goes on pin 2.

the 1M grid leak resistor on the input is usually soldered directly across the input jack too. oh, not sure if you know, but make sure you use a switching jack, not a guitar jack for the input, ie. the input line is earthed when you pull out the guitar lead.

looks good, and you look ready to crank up that iron!

e

Permalink Reply by GARAGE HERMIT on October 4, 2012 at 12:49am

thank's for the info eric, could'nt have done it without your help, i've been itching to start soldering, but i wanted to make sure i had the schematic right before i started, and also trying to understand how a valve and valve amp's work, i've gained a bit of knowledge, but i've still a long way to go, but i'm really enjoying it,

Permalink Reply by GARAGE HERMIT on October 5, 2012 at 11:49pm

started putting a test bed together, just to see if everything work's,

 

Permalink Reply by eric gittins on October 8, 2012 at 12:37pm

hey, hope the build testing is progressing well. was just looking through some old schematics i have here, and found this for the fender harvard.

its the first i've seen with your 6at6, and the cathode res and cap and plate voltages for the preamp would be a good place to start for a guitar amp.  as i mentioned before, i would also put in a one knob tone control.  ones like in this circuit use no extra parts on the board - the 2 caps and the wiring all just run between the tone and volume pots. very simple, and works too.

e

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