PA amp to guitar amp convertion.

[distortionhead]

Got this old pa amp with 6x EL84 . any idea if it can run on any other power tube , like 2 EL34 or 6550 ? main criteria is I dont want it to run on EL84 want to change to some different tubes.

[sluckey]

You'll need to change the bias supply. Probably need a different PT and OT. May be simpler to just scratch build what you want.

[distortionhead]

yeah i was thinking of scratch building a new amp reusing the PT & OT.

[sluckey]

Post a big pic of the schematic so we can take a closer look.

[trobbins]

Are EL84 / 6BQ5's a problem for you somehow?

The amp must be fairly 'modern' given 1975 on the drawing, so perhaps it doesn't need much checking to get working, given that parts and wiring may all be quite ok.  There's lots that can be easily done to modify  the early gain stages to make it suitable for guitar use, as well as to improve the ability to overdrive the output stage.

[PRR]

What's wrong with EL84? It is a fine tube. It is your best bargain power tube.

[distortionhead]

just wanted to try a different output section.

[shooter]

start with figuring out the OT, with that info you can pick other tubes that can meet those spec's
so impedance is a great starting point, sans that, solve for the turns ratio, then do the math for impedance.


the already visible "limiting factor" is the B+ to the tubes.  Low 300's will give you a very different sound on say KT88's than >400vdc B+ will

[kagliostro]

About the migration to EL34 tubes ... why ?

Lot of people say EL84 sonds very close, only less power

Some idea for your conversion

Weber Kit & MAC 6C old italian amp ('70)









Franco

p.s.: If you can post it, also I'm interested to have a higher definition image of the schematic of your amp

[trobbins]

The thought of changing from a sextet of EL84 to another tube configuration seems to be quite technically involved in order to work through all the detailed consequences.  Imho there would be quite a risk of a lot of effort ending in disappointment, especially without a good technical plan mapped out beforehand.  If it was me then I'd look around for another cheap PA amp with the target output stage valves to start with.

[kagliostro]

Conversion ..... what about a partial conversion ?



also this will be a possible solution to your request

each EL84 requires 0.75A of current for heaters, each 6V6 requires 0.45A

so there is no problem to swap tubes (you will have a surplus of 0.60A for a pair of tubes)

About B+ el84 can afford a max voltage of 300V and 6V6 around 350V, so no problem also here


obviously you must swap a pair of noval sockets for a pair of octal


to use 6P1P will avoid the use of the octal socket (6P1P is a nine pin tube) but the max B+ must be 250V and also, this days I see 6P1P unpopolar


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I think that your OT primary impedance is around 3K (you can do a bit of math and see which it is), 3K will be good also for a pair of other tubes, but why mess with a total Power Section swap ? You have a working Power Section, use it !

Franco

[distortionhead]

Oh sorry I didn't mention that the EL84 didn't come with the amp. That is why I was thinking of redoing the output section

[mresistor]

Oh sorry I didn't mention that the EL84 didn't come with the amp. That is why I was thinking of redoing the output section

Do you mean the amp didn't come with a sextet of power tubes?  Might not be easy to find some now.  Or any tube for that matter. The Mesa Boogie Blue Angel is interesting.

[distortionhead]

Oh sorry I didn't mention that the EL84 didn't come with the amp. That is why I was thinking of redoing the output section

Do you mean the amp didn't come with a sextet of power tubes?  Might not be easy to find some now.  Or any tube for that matter. The Mesa Boogie Blue Angel is interesting.

.

Yes

[kagliostro]

Today

https://tubedepot.com/products/electro-harmonix-el84-eh-power-vacuum-tube

https://tubedepot.com/products/sovtek-el84-power-vacuum-tube

Franco

[mresistor]

Today

https://tubedepot.com/products/electro-harmonix-el84-eh-power-vacuum-tube

https://tubedepot.com/products/sovtek-el84-power-vacuum-tube

Franco

Franco   did you look to see if they were in stock and for sale?

[kagliostro]

They are NOT disposable at the moment as mresistor say, not easy to find

may be is possible to find something among the high price tubes, not the low cost

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https://tubes-store.com/advanced_search_result.php?keywords=6p1p

https://tubes-store.com/advanced_search_result.php?keywords=6p14p

Franco

[mresistor]

*deleted*
Don't think he is going to find a sextet anywhere    quad  maybe at Viva tubes . 

[distortionhead]

also im from india, shipping and import duties are high.

[AmberB]

Right now, I have a bag full of EL-84s, probably 30 of them.  I don't know their condition.  They're all used, I got them as part of a pile of tubes from a closed repair shop back in the 1980s.  I've never had any reason to test them to see how many are good...

I also have 20-30 6GK6s, and probably that many 6FQ7s, but that's a different story...

[AmberB]

With that amp normally runinfg 6 of the EL-84s, there's enough heater power for a pair of EL-34s.  Of course, the primary impedance of the OT may not be suitable for a pair of EL-34s.  With multiple taps on the output side of the OT, you may be able to find a combination that works for the EL-34.  Of course, you'll have to put in a pair of octal sockets for that.

[kagliostro]

EL34 Heater consumption is 1.5A for each tube, with the B+ we can see on the schematic and a load around 3K - 3.8K you can arrive (with a pair of tubes) around 35 - 40 W

As to give further councils, I see the necessity to have indication about the PT voltages and OT primary impedance

Otherway, we can go on discussing about this or those or that tubes, but only talk

---

A bit more readable version of your amp schematic

(if you have e better version, please post it)



Franco

[distortionhead]

here is a scanned copy of the circuit

[kagliostro]

Thanks for the scan



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On the schematic the Bias voltage is 17.5V (then it is drop via a resistor to 13.5V)

so if you want to change the output tubes you must consider this voltage (17.5V) or go for a Cathode Biased Power Section

Franco