Diezel VH4 running very hot with KT88s

[chris_1984]

Hi,

this is my first post on this forum, so: hi guys! i bought my VH4 new in 2008 and recently got it 'updated' by peter last year in october. unfortunately, one of the TAD EL34 dies briefly after, so i decided to try out a set of JJ KT88/6550 powertubes.

anyway, after installing and biasing them i noticed that they get QUITE hot. i'm aware they have a significantly larger dissipation rating than 6L6s or EL34s which is causing this, but i was just wondering if it's 'safe' to run the amp like this. i don't remember the exact value, but after measuring the plate voltage on my VH4 and checking the KT88's spec sheet, i biased them quite cold at around 45-50mA per tube.

yesterday after band rehearsal the amp was running for about 3-4 hours total. the back grill of the amp was so hot behind the tubes that i wouldn't want to keep my fingers on it for more than 1 second. the top of the headshell was also quite hot.


so, to make a long story short: is it safe to run my VH4 with a quad of JJ KT88s?


thanks,
christian

 

[Peter Diezel]

Dear Christian,

I stopped to recommend the big bulbs long ago.

You told the reasons and I am not convinced
sound wise. We used 6550 for a short time and
had lots of issues with the glas, damaged by
the movement of the spring retainer.

It´s a matter of taste but the KT77 JJ is the most
reliable EL34 type on the market this time.

Best,
Peter

 

[chris_1984]

thanks for the input, peter.

i'll give KT77s a try

 

[Peter Diezel]

Welcome to the forum btw

 

[chris_1984]

Welcome to the forum btw

thanks

 

[chris_1984]

still, i have to ask again. tube reliability taken aside: am i risking damaging the amp by running KT88s?

the reason i'm asking is because i'll be using the amp for rehearsal/shows during the next weeks until i'm able to swap tubes.

 

[Rødskjegg]

still, i have to ask again. tube reliability taken aside: am i risking damaging the amp by running KT88s?

the reason i'm asking is because i'll be using the amp for rehearsal/shows during the next weeks until i'm able to swap tubes.

I once fitted my Herbert with KT88's, shortly after that I needed to send the amp to Peter to get it repaired. I hope that answers your question

 

[chris_1984]

I once fitted my Herbert with KT88's, shortly after that I needed to send the amp to Peter to get it repaired. I hope that answers your question

thanks for the input. but i'm afraid this does not really answer my question. running a sextet of KT88s puts a much higher load on the PT than running a quad. also, i'm assuming that the herbert utilizes a different PT than the VH4.

still, sucks to hear that. i'm sure peter took good care of your amp, though

 

[Jimbo72]

Just had Jens retube my VH4 with Gold Lion Genalex KT77's and it sounds KILLER!!! I highly recommend the Gold Lion's.

 

[bananaladonpcp]

Just had Jens retube my VH4 with Gold Lion Genalex KT77's and it sounds KILLER!!! I highly recommend the Gold Lion's.

The new Gold Lion tubes are pretty bad ass!!

 

[chris_1984]

ok guys, thanks for the input guys.

i'll by swapping the KT88s for some TAD EL34B-STRs, JJ 6L6s and Sovtek 5881s i have left.

i'm also intrigued by the KT77s. how do these compare to the rest of the bunch?

 

[MourningEngine]

Keep in mind that you cannot mix output tubes with the VH4. Only use matched quartets of EL34s, 6L6s, KT77 etc.

 

[chris_1984]

Keep in mind that you cannot mix output tubes with the VH4. Only use matched quartets of EL34s, 6L6s, KT77 etc.

yeah, i'm aware of that

i see how one could misinterpret my last post, though. what i meant to say is that i have a quad of each tube type to try out

 

[chris_1984]

just wanted to give a little heads up on this:

i've biased the KT88s a bit colder to 40mA and have been running it like this without issues since then. I know, the 'big bottles' are not recommended by peter, but i simply prefer them in this amp by a long shot over EL34s. The amp also doesn't heat up as bad with the tubes biased colder. 5-10mA per tube can make quite a difference here it seems

 

[Ventura]

just wanted to give a little heads up on this:

i've biased the KT88s a bit colder to 40mA and have been running it like this without issues since then. I know, the 'big bottles' are not recommended by peter, but i simply prefer them in this amp by a long shot over EL34s. The amp also doesn't heat up as bad with the tubes biased colder. 5-10mA per tube can make quite a difference here it seems

Careful. We have people everywhere.



Uncle Mo

 

[chris_1984]

just wanted to give a little heads up on this:

i've biased the KT88s a bit colder to 40mA and have been running it like this without issues since then. I know, the 'big bottles' are not recommended by peter, but i simply prefer them in this amp by a long shot over EL34s. The amp also doesn't heat up as bad with the tubes biased colder. 5-10mA per tube can make quite a difference here it seems

Careful. We have people everywhere.



Uncle Mo

 

[shadowdevourer]

Now I wanna try them too on my Herbert

 

[kahawe]

Careful. We have people everywhere.



Uncle Mo

Did someone say "CLAMP HIM!!!" hm??? ;D

 

[chris_1984]

Did someone say "CLAMP HIM!!!" hm??? ;D

futurama FTW!


yeah, i'm already getting a pair of 6550 so i can also start experimenting with my Herbert (currently all EL34s)

putting four KT88s and a pair of EL34s already changed the Herbert quite a bit. A lot more than preamp tube changes. The KT88s definitely make the amp sound less compressed and more dry.

 

[Flavatrocious]

SED 6550's seem to be the tube for me in my VH4. Makes the amp come alive. I'd like to try KT88's but i'm scared!!!