Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frank57 on June 14, 2011, 01:53:47 pm

Title: What happens when you raise or lower these resistors on the power tubes?
Post by: frank57 on June 14, 2011, 01:53:47 pm

I haven't found a good text the ins and outs of the power section screens and grids and so on.

But say on this Hiwatt as an example,if I raised r31 and r32 to 1k and r44 to 2k2 would you get more power tube distortion sooner? Any negative side effects?
I was also wondering about r29 and r30 what happens when you raise or lower the values there?

Title: Re: What happens when you raise or lower these resistors on the power tubes?
Post by: kagliostro on June 14, 2011, 02:11:24 pm

Changing r31 & r32 (increasing it) will sure give more sag

see the Mesa Boogie attached schematic

they used a switch to insert and exclude this feature

Kagliostro

Title: Re: What happens when you raise or lower these resistors on the power tubes?
Post by: frank57 on June 14, 2011, 03:21:45 pm

So it will have more of a compressor like effect  making the power tubes appear to break up sooner?
Would it cause the voltages to rise?

Title: Re: What happens when you raise or lower these resistors on the power tubes?
Post by: archaos on June 14, 2011, 03:49:03 pm

Hi, upping screen-grid resistors from 100 ohms to 1k, 2k2 or even more for instance turns out to be a good idea. Particularly as regards EL84s. Larger screen resistors impart better touch response & imbue the amp with a louder sound @ all levels. The sound characteristcs of the amp when played loud occur @ a much lower volume. Tube life is lenghtened too.

Larger grid-stops for g1 roll off highs.

Do have a look to the screen resistors thread attached.  :icon_biggrin:

Title: Re: What happens when you raise or lower these resistors on the power tubes?
Post by: jjasilli on June 14, 2011, 04:22:17 pm

I tried large screen resistors in my PP el-84 Traynor Bassmate (no screen resistors stock) - they destroyed the overdrive tone IMHO.  OTOH, the plate voltage is way high about 410 VDC, which might be an issue.  I've finally decided to use a bucking tranny to knock down the supply voltage, but that's for another thread.  Anyway screen resistors from 100R to 470R are pretty common to protect the screen; not sure if they affect tone much.  The larger resistors, 1K and up, do affect tone.  Your mileage may vary, oh what the heck: YMMV!

Title: Re: What happens when you raise or lower these resistors on the power tubes?
Post by: frank57 on June 15, 2011, 12:07:37 pm

Sounds like it might be worth a shot in this 2 stages version which is super clean.
The voltages are also pretty high and not much drop to the screens .
Some simple samples of what it sounds like are here:
Single note test going between pickups nothing fancy.:
http://www.mediafire.com/?x2m8o33s3af31ra (http://www.mediafire.com/?x2m8o33s3af31ra)

Chords:

http://www.mediafire.com/?0f4ela4im33a6eb (http://www.mediafire.com/?0f4ela4im33a6eb)
But you really have to whack it pretty hard with the gain on 10 master 3 to get it to distort.
Everything else at high noon.
So in this version of the amp it's behaving very much like a Hiwatt circuit.
You can actually play clean very easily without touching your volume knob.