Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on October 28, 2011, 05:22:28 am

Title: Grid leak on KT88
Post by: LooseChange on October 28, 2011, 05:22:28 am

It looks like I should change the grid leak on the power tubes when going from EL33's to KT88's.
Do I need to change them from 220k to 100k in a fixed bias amp?

Data sheet below (sorry bad quality)
http://www.tubezone.net/pdf/kt88new.pdf (http://www.tubezone.net/pdf/kt88new.pdf)

Thanks!

Title: Re: Grid leak on KT88
Post by: stratele52 on October 28, 2011, 06:47:08 am

I see some amp KT88 schematics using a 220 k

Title: Re: Grid leak on KT88
Post by: DummyLoad on October 28, 2011, 01:44:56 pm

Single pair use 100k. More than 2 pairs 47k.

Title: Re: Grid leak on KT88
Post by: LooseChange on October 28, 2011, 04:02:28 pm

I am converting a JMP Master Lead 100watt amp.
Change them to 47k?

Thanks!

Title: Re: Grid leak on KT88
Post by: DummyLoad on October 28, 2011, 04:20:14 pm

Quote from: LooseChange on October 28, 2011, 04:02:28 pm

I am converting a JMP Master Lead 100watt amp.
Change them to 47k?

Thanks!

in the now i say i would. i need to look at schema - you may need to re-think PI as well. KT-88s will need more swing to g1 and with 47K load PI may not cut it, but then again you may not want full power from 4 x KT88 in this app. with 4 KT88's you'll be putting a strain on the PT and OT if there were 4 x EL34 previous, but i'm sure you're aware of that though.

if it were mine, i'd use 2 KT88s, 100K grid-leak and leave 2 holes open - nothing changes much but bias and perhaps tweaks on PI to make more swing.

peace.

--DL

Title: Re: Grid leak on KT88
Post by: HotBluePlates on October 29, 2011, 01:06:06 am

Keep in mind that the recommended grid resistor has to do with the amount of cathode current, and the likelihood of grid current due to that and residual gas. The goal is to keep the bias stable in operation.

Note that the GE 6550A data sheet recommends 50k max per tube for fixed bias (6550 data sheet (http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6550A.pdf)).

When you reduce the grid resistor, you will need to increase the coupling cap between phase inverter and output tubes to maintain the bass response.

Additionally, the lower grid resistance is a heavier load on the phase inverter, and will reduce the gain of the PI. You may have to revisit the design of the phase inverter, depending on the particulars of your amp. That's in addition to any larger phase inverter output required by a greater bias voltage, if that occurs in your amp.

Title: Re: Grid leak on KT88
Post by: LooseChange on October 29, 2011, 07:02:15 am

What would you recommend to convert the JMP MK2 Master lead 100w amp?
Thanks!

Title: Re: Grid leak on KT88
Post by: ernie_jr on October 29, 2011, 09:13:46 am

check out the sunn 2000s  4 kt88 (or 6550) and 100k grid leak resitors
Ernie