Too much B+ for an EL84?

ik6gpy · Post by **ik6gpy** » 01 Dec 2008, 17:42

Hi there!
I'm going to build a simple Ax84 Hi-Octane version, but i've got in my hands a 260V-0-260V (40mA+40mA) Power tranny (instead of the 190-0-190 suggested for this project). A solid state rectified 260V gives me approximately 365V on the B+ tap (i'm using the Duncan Amp power supply software for the simulation) . Isn't it a bit too much for an EL84?
Could i lower B+ voltage with a resistor in series after the rectifier, without loosing too much current ? Any suggestions?
Sorry for the (stupid) questions but this is my first tube amp build. I'm still learning

Thanks in advance!
Matteo

KindaFuzzy · Post by **KindaFuzzy** » 01 Dec 2008, 18:03

That's probably ok for EL84's, but I tend to play it safe and not run them with that much voltage. This is just my opinion, but EL84's sound pretty bright and harsh with higher voltages. I've had a couple of amps where too high a voltage was a problem and have tried just using a tube rectifier, or switching to 6v6's if you haven't built it yet, they can take that kind of voltage pretty well. The other trick is to lower the B+ with a power mosfet, like on this page: http://www.geofex.com/Article_Folders/m ... tfolly.htm

I tried it and it works like a charm, but the Mosfet does get hot, so you'll need a heat sink. Scroll down to the bottom.

ik6gpy · Post by **ik6gpy** » 01 Dec 2008, 18:13

Thanks a lot! That's exactly what i was looking for. I'll try that. Thanks again
Matteo

mojah63 · Post by **mojah63** » 01 Dec 2008, 20:06

An inductive power resistor like 25W would work too. Try 200 to 400 ohms, plug it in Duncans program and see. They come with a heatsink... My classic 30 has high B+, with the el84's biased with enough current it doesn't sound harsh at all. Be careful not to exceed the power rating.

ik6gpy · Post by **ik6gpy** » 01 Dec 2008, 20:56

The power resistor thing was my first idea.
I'll try both the zener and the power resistor options and i'll go for the less noisy configuration. Thanks for the reply!
Matteo

chris_d · Post by **chris_d** » 01 Dec 2008, 22:42

I don't know, to be within spec for the High Octane, with that PT, you will need to drop something like 100v? That is quite a lot to ask of a resistor solution. I personally wouldn't use the mosfet solution for more than 30-40v. You are going to need some serious heatsinking. I personally think that it makes more sense to spend $35 on the right transformer for the job, and save the PT you have for some other project. For instance, these ones look good: http://www.musicalpowersupplies.com/4.html

For the record, for a single EL84, 365v is way too much. For a pair of EL84s it is high but useable, but not for just one.

-chris

ik6gpy · Post by **ik6gpy** » 01 Dec 2008, 23:40

Yes, buying a suitable power transformer would be the smartest thing to do, but here in Italy an Hammond 369EX costs something like 80$ (63€)...

I even thought to switch the El84 with an El34 and redesign the power supply, but in this case i'm not shure if my 80mA tranny would be "strong" enough to function properly.

Do you have in mind some "starters" build suitable for what i've got in my hands?

Many thanks in advance

Matteo

tunghaichuan · Post by **tunghaichuan** » 02 Dec 2008, 00:46

Take a look at the way the power supply is configured in the Valve Junior amp:

http://www.diycustomamps.com/images/sch ... _schem.pdf

The first node of the B+ (C6) is not connected to to the tubes. It is decoupled to the next node (C9) of the power supply with a 220 ohm, 1 W resistor (R10). The Valve Junior uses a 260-0 PT and has a 115v primary for use in the US. The plate voltages are usually too high with the stock resistor as the line voltages are well above 120v. Substituting a 1K ohm or even 1.5k ohm resistor for the stock R10 value is a common mod to get the plate voltages under control. I would use a 2-3 watt rated resistor for this position.

tung

ik6gpy · Post by **ik6gpy** » 02 Dec 2008, 09:08

That looks very simple, thanks a lot!

JiM · Post by **JiM** » 02 Dec 2008, 23:26

I've used a very simple trick to lower a bit the B+ on my Valve Jr. copy : use another primary tap on the PT.

It may not work for your specific transformer, but some have several primary taps for different countries : 100V for Japan, 120V for USA, 230V for Europe and 240V for Australia & New Zealand (is that right ?)
Living in Europe with a correct 230V on tap, i've wired the PT with the 240V tap to lower all the internal voltages by 4.3%. That's within specs for the heaters, and give some fresh air for the EL84.

Sorry if that doesn't help in this case, but i thought it might give some ideas to Japanese and European people with too much voltage, or American and Upside-Down people* with not enough voltage !

By the way, would it harm the PT if you only use one half of the secondary, i.e. 260V-0 and leave the second 260V tap open ? I know it's not meant to be used like that (and it will be limited to only 40mA), but it's the same with the primary at 120V, or with the OT secondary if you use the 4 Ohm or 8 Ohm tap compared to resp. 8 Ohm or 16 Ohm. Am i saying something silly ?

* No offense for Aussies and Kiwis

i just couldn't resist

ik6gpy · Post by **ik6gpy** » 03 Dec 2008, 00:12

Well, increasing the primary tap to 240V when you got a line voltage of 230V will result in little less secondary voltages, it's just like a Variac effect. But not all the transformers have got multiple primary taps...
Anyway i don't think that using only one 260-0 tap would be a problem (I'm i right?). The real problem will be the limited current (40mA in my case), maybe it's too low to power a single El84 + 2 x 12Ax7 ?

At the end I think i'll use the "classic" Hi-Octane supply circuit and lower it with a 50V zener, plus i'll use a variable Bias. This will result in a 315V B+ (approx) and should be safe enough for the tube, as the max suggested B+ for an El84 is 350V.
Thanks for all the replies guys!
Matteo