• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

question about swapping el34 for kt77 in my tube amp

johnzm

Member
2010-07-14 8:43 pm
I am doing some research on a tube amp I just aquired, and need some help from people a bit more intelligent than iam, so that i do not blow it up!

the amp is an onix H34. the tubes are audo biasing EL34's.

i would like to roll KT77 into it, but i am not knowledgable enough to know what this could effect/destroy, if i do.

does anyone have any thoughts on the possible problems or issues i might run into, or know if this will work for me?

Thanks!
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
I am doing some research on a tube amp I just aquired, and need some help from people a bit more intelligent than iam, so that i do not blow it up!
the amp is an onix H34. the tubes are audo biasing EL34's.

i would like to roll KT77 into it, but i am not knowledgable enough to know what this could effect/destroy, if i do.

does anyone have any thoughts on the possible problems or issues i might run into, or know if this will work for me?

Thanks!

Just for info,

Remember voltage is high on a tube amp so take care! Have you worked on tubes before? If you are OK from a safety point of view then:-

I have not seen the circuit for this amp (does it have resistors on the cathode of the EL34's), if you can measure a voltage on the cathode resistors to ground you can work out the current when biased! If you do try the KT77 then you could see what the bias current is! I have tried many tubes in a fixed bias amp meant for EL34's, however I biased each manually with pots.

One rule of thumb is if the anode starts to glow red then you are going to damage the amp due to over current! So watch for anything glowing other than the heaters!

Any tube you try will need a different bias to set the "idle current" in the tubes, so you are taking a risk if you do not monitor the current in the tube!

You will find if all goes O.K. that it will depend on the type of music you listen too! I found the EL34 gave more weight to the bass the KT type seemed to be tighter.

Regards
M. Gregg
 
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mace1337

Member
2005-08-21 11:21 pm
Thanks for you input.

this will be the first tube i will roll in, since alot of talk about the EL34 being mostly known for a lack of tight sound.

does anyone have a short list of what other tubes should roll easily besides the el34/kt77?

Some tubes to try:
6L6GC
Russian 6P3S-E with the coin base, they're cheap and worth looking at.
5881 which is like the industrial version of the 6L6

You could try KT88 or 6550 too.

In all cases, be sure that plate voltage does not exceed the ratings, and that the bias is set correctly or you are in for some very expensive fireworks.

edit: You may find this interesting: http://www.jacmusic.com/KT88/kt88.htm
 
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CHECK THIS LINK OUT

George who is a major contributor here and to the DIY community with designs and experiments has posted this chart with some simulations of different B+ voltages and bias points with some of the more common octal power tubes.

His site is a wealth of information for people interested to learn more about tube amps and their basic operation - with a great emphasis on SAFETY.

You would basically line up your B+ voltage, tube type, OPT impedance and then pick your cathode resistor value to reach the bias point you want to try.

These charts are for Single Ended operation FYI...

Not sure if your amp is SE or PP...
 

20to20

Member
2010-06-23 9:25 pm
if you can give me some general #'s i can measure it and verify its going to work ok. i am not opposed to probing around inside of it, and was actually gonna pop the top to check it out before i turned it on the first time.

If your amp doesn't give you a bias adjust control and guidance for how to set it, you'd be better off, at first, just rolling EL34's of different brands for fun and experience. Until you've read some tube data sheets and understand the differences between different tube types, you'll just be "sticking in" different tubes and hearing how your amp responds without understanding why the volume or frequency response has changed and then you will be posting the same kinds of statements about those tubes that you've read and repeated about the EL34.
 
If you are thinking of putting JJ made KT77 into your amplifier I would make sure to purchase a set that have been burned in for 24 hours minimum and then matched. Currently manufactured JJ octal power tubes do not have the best reputation for reliability.

Tell me about it! Lost 6 JJ E34L's in 5 months in a PP amp back in 2005. Slapped in some winged "C" Svetlana"s, been crankin' ever since. ;)