OTL guitar amp?

[sean k]

I just picked up an old phillips radiogram for 65 bucks that works well and I had pulled one of the output tubes while it was in the shop and it was an ECL86... there were what looked like a quad so I bought it even though I couldn't see any OT's... there aren't any! I've only just pulled it out of the box and theres two ECl86's and two EL86's and no OT's. Quick look on the net and it's output tranformer LESS, OTL, and has two 800 ohm speakers.

I'll look a little deeper into it but my guess is the ECL86's are voltage amps and current drivers then into the EL86's to add even more current... Should be an interesting guitar amp!

[Michael]

I have no idea what you just said, so I'll post this instead:

[sean k]

Well the basic premise is that the output transformer converts high voltage and low current into low voltage and high current... so that the tubes, at maybe 5000 ohms resistance, can drive a speaker of 8 ohms resistance. But if you make a speaker with super fine wire, like a guitar pickups, and lots and lots of windings then you can get it into a region where a bunch of current amps will be able to drive it.
I haven't looked at it yet but my guess it's ordinary voltage amp stuff to the output of the ECL86's plate and then the EL 86 is setup as a cathode follower so it's pushing current as opposed to voltage and the output goes through a cap to the high impedance speaker... but I'm only guessing so far and will trace it through and figure things out.

Now if whe've got any radio buffs here can someone tell me an easyish way to convert the radio part to shortwave? Or am I completely wrong about radios and a different approach is required.

[tubeswell]

my guess is the ECL86's are voltage amps and current drivers then into the EL86's to add even more current...

The ECL86 (or 6GW8) is just a normal triode and pentode in one envelope. The triode is similar to 1/2 a 12AX7 and you can hook a pair of the pentodes up to a PP OT with an 8k primary (or a quad to a 4k OT). You can use the triode parts of each tube for the different sides of a LTP (or other) phase inverter. There's a few designs of guys that have built similar amps using ECL82 (6BM8) over at the Amp Garage. (But the ECL86 can handle a higher plate voltage than the ECL82). Also check out the SE amp (Low Cost Amplifier) using ECL86 on the link on this page.

http://r-type.org/exhib/aaa0034.htm

The EL86 is like an EL84 but with a low plate resistance (2k5), so running 2 x 800R speakers in parallel (400R) without an output transformer is within the ballpark of a load being somewhere between 1/6 and 1/8 of the tube's plate resistance.

But I would tend to think this mightn't be much good for geetar amps, because I can't see how you'd be able to get push-pull without an output transformer (because for PP you need to convert opposing signals back into one signal again for the speaker(s)), so you'd be limited to SE - which is hummier. (Ed - I had some further thoughts about this under the cold hard light of day so I changed my tune)

[IkeKrull]

This looks quite relevant - some info about the transformerless design. and it will probably use an electrodynamic speaker.

http://www.instructables.com/id/How_to_ ... ube_radio/


But be careful - apparently many of these amps chassis are live when they are powered up, according to some stuff i read you should only attempt to mess around with them if you have it plugged into an isolating transformer.
'

http://www.tdpri.com/forum/shock-brothe ... -amps.html

[sean k]

I haven't as yet gone into it properly as I'm still trying to sort out the signal input into it. I know it's pretty straight forwards as theres just the two triode grids on the ECL86's to track back from but they did some weird stuff on these old grams. It is indeed two single ended amps but it's also like a regular transistor amp with big electrolytics coming off the output... which are definitely the cathodes of the EL86's.

I'm not so interested in the guitar amp sound persay but am interested in this as a clean amp for amping piezo signals and possibly even microphones.

I was vaguely hoping I could change the radio to shortwave then have a ring modulator between the shortwave output, radio as carrier, and the gram input.. which with a piezo needle pickup is already suited to very high impedance except for any RIAA eq... but it ain't easy to go shortwave so I'll just make a plain amp.

You mention 8k for EL84's. How about 6.6k?, or is that a little low... would need a much lower B+ I suppose.

[tubeswell]

Anything between 5k and 10k will work, with anything between 6k6 and 8k5 being more 'optimal' for the load resistance.

Don't know much about radios sorry.

[Rog]

Is the radiogram anything like this one?
If so, it had a nice sound for what it was (as a geetar amp) almost 50 years ago...

[tubeswell]

Is the radiogram anything like this one?
If so, it had a nice sound for what it was (as a geetar amp) almost 50 years ago...

Really Rog? So that's you 50 years ago?

[Polar Bear]

Is the radiogram anything like this one?
If so, it had a nice sound for what it was (as a geetar amp) almost 50 years ago...

Really Rog? So that's you 50 years ago?

That's him 80 years ago, he's photoshopped himself into a photo of his Radio from 50 years ago.

[sean k]

I'd say mine was from the sixties, late or maybe even the late seventies. 6.6k cool! I've got a 325-0-325 there and some big chokes so maybe a choke input with a .22 - 1uf just off the diodes to pull up the voltage over .7 of AC.

[tubeswell]

I'd say mine was from the sixties, late or maybe even the late seventies. 6.6k cool! I've got a 325-0-325 there and some big chokes so maybe a choke input with a .22 - 1uf just off the diodes to pull up the voltage over .7 of AC.

You can use the 325-0-325 with a standard CLC filter, if you have it tube rectified and put chunky limiter resistor in series with each side of the HT winding and the rectifier's anodes - to lower the rectifiers output. (See Merlin Blencowe's article on limiting resistance for FW rectifiers here: http://www.freewebs.com/valvewizard/fullwave.html

[Rog]

Really Rog? So that's you 50 years ago?

Indeed and I'm just as slim today!!

[Rog]

That's him 80 years ago, he's photoshopped himself into a photo of his Radio from 50 years ago.

By the cringe, when I get to the North Pole you are so gonna get wasted....

[sean k]

Tubeswell, I'm not really a fan of big power dropping resistors and I don't have a 5V winding on the PT to drop in a tbe rectifier, but if I was to drop the voltage first I'd most probably have zeners on the CT but I like my little trick of using a small signal type cap on the front of a choke as you can tailor the voltage you want anywhere from just over .7 of the AC voltage right up to x 1.4. I first did it years ago and can't remember if someone suggested it or I figured it out myself but it seems like a good way to use a biggish choke.

I'll have to look at some data sheets on the EL84's and ballpark the voltage I'd want with a 6.6k OT.