pulling 2 EL34's

[blue]

evenin boys

i've just been asked a question i don't know the answer to, and i immediately thought of this place! :)

a friend of mine wants to buy a Bugera 1960, a clone of a Marshall 1959 plexi.  he wants to know is it safe to pull two of the el34's to halve the output, and if so which two, and would the 8 and 16 ohm outputs still be 8 and 16 ohm.

i haven't a clue myself, hope someone can advise.

[HTH AMPS]

yep, its perfectly safe - pull the inside pair OR the outside pair. 

then set the amp's impedance switch to half of the cab's impedance.  e.g. 16ohm cab, 8ohm on the amp.  OR, 8ohm cab, 4ohm on the amp.

[blue]

thanks very much mister Hell :)  i'll pass that on

[Keven]

tried and true. been running on that setup for almost 2 months now with my traynor. works great. :D

[MrBump]

Is it always the case that you need to change the impedance if you pop a couple of EL34s out?

I only ask because that's how I was running my Orange for years, but always at 8ohms - the same as with the full compliment of 4.

Doesn't the OT dictate the output in terms of ohms?

Mark.

[Dmoney]

the OT sees an impedance from your output valves.
HTH is right.
If you pull two tubes you should set the impedance to half the value of your cab.

if you were using your head set to 8ohms, minus two tubes, with an 8ohm cab, i dont think thats right.

[James C]

+1 to the impedance confusion,

I'm using a JVM410H that the previous owner pulled two tubes from, but there is no impedance switch, just a selection of speaker outs of different impedance, so i just run from the 8 ohms output of the amp to the 8 ohms input on the cab.

Haven't had any probs, but don't want to have any either so advice is welcome

[blue]

he has another, related question!

he's been told that replacing the output transformer will transform (no pun intended) the tone of the amp, bringing it much closer to a real 1959.  it was my thought then, would it be possible to put in a 50 watt OT, and effectively convert the whole amp to 50 watts?  then, i presume, the 16 and 8 ohm taps would be correct.

[HTH AMPS]

he has another, related question!

he's been told that replacing the output transformer will transform (no pun intended) the tone of the amp, bringing it much closer to a real 1959.  it was my thought then, would it be possible to put in a 50 watt OT, and effectively convert the whole amp to 50 watts?  then, i presume, the 16 and 8 ohm taps would be correct.

those transformers won't be the best at that price, so yes, upgrading the OT will make a big improvement in tone.

do they make a 50w (Bugera) version? - would seem better to buy the correct wattage amp.  simply replacing the OT with a 50w one and running two EL34s still won't make it exactly like a 50w plexi as the 100w amps have different filtering and higher plate voltage too.

if he wants a 50w plexi tone, get a 50w amp.

[Frank]

Is it always the case that you need to change the impedance if you pop a couple of EL34s out?

I only ask because that's how I was running my Orange for years, but always at 8ohms - the same as with the full compliment of 4.

Doesn't the OT dictate the output in terms of ohms?

Mark.

The OT is really there to match the high impedance of the valve circuit to the low impedance of the speakers. Popping two output tubes out changes the impedance in the plate circuit so you do need to change the impedance seen by the transformer secondary.

[MrBump]

Is it always the case that you need to change the impedance if you pop a couple of EL34s out?

I only ask because that's how I was running my Orange for years, but always at 8ohms - the same as with the full compliment of 4.

Doesn't the OT dictate the output in terms of ohms?

Mark.

The OT is really there to match the high impedance of the valve circuit to the low impedance of the speakers. Popping two output tubes out changes the impedance in the plate circuit so you do need to change the impedance seen by the transformer secondary.

OK...  so how come my Orange works on 8 ohms with either 2 or 4 EL34s?

Confused...

[hunter]

It's not always inside/outside pairs. My Steavens and 2203 both have 2left 2right pair arrangements, and the 100B has 1&3 and 2&4.

[Frank]

OK...  so how come my Orange works on 8 ohms with either 2 or 4 EL34s?

Confused...

it'll still "work" as in it'll produce sound but it won't be running at optimum efficiency, exactly as if you select the wrong speaker impedance on the amp. You're also running the risk of damaging the tubes or the OT by doing this.

Here's a ROUGH guide to what's happening: the tube circuit needs to work into a high impedance, say around 4,000 ohms. The speaker is low impedance, around 8 ohms. The ratio of turns between the primary (plate circuit) winding and the secondary (speaker) winding of the OT determines what impedance is "seen" by the tube circuit for a given speaker impedance.

Popping two tubes means the primary impedance changes. If you keep the same impedance on the secondary (i.e. you don't change the speaker impedance selector) then the impedance RATIO is incorrect and you're effectively running your amp with the speaker impedance set wrong.