Phase Inverter Questions

[kd]

I haven't seen any recommendations for using the Dale resistors on the phase inverter plates. Is a carbon film normally used in that position?
Also, It seems that the Sovtek 12ax7LPS is one of the more popular phase inverter tubes. Should I get a balanced or unbalanced tube?

Thanks,
Ken

[heisthl]

AFAIK the PI uses metal films - just not the V/D ones.

Even with the balance trimmer on the PI plates a lot of the tubes won't get close enough so it's worth the money to get a balanced one.

[David Root]

"Normally" yes, but that doesn't mean you can't use other types if you want.

My reason for using carbon films here would be to get a little even harmonic distortion going in the PI. Don't know if HAD used them there or not, but the plate voltage is high enough that a half watt carbon film should show a little 2nd harmonics anyway.

The PI circuit itself is not going to generate a lot of harmonic distortion because HAD's design centers it pretty much on the load lines.

[kd]

Thanks fellas, much appreciated.

Ken

[CHIP]

What brands of metal film resistors are prefered? I once read to stay away from xicons.

[Structo]

For low tolerance MF resistors many of us here like the Vishay/Dale RN series resistors.
Military spec and they work well as plate resistors because they don't drift with age or heat.

http://www.mouser.com/catalog/catalogUSD/640/655.pdf

[talbany]

The main reason I like them as plate resistors is their ability to cut down on the Hiss when you start adding some serious gain to the OD...They are quiet!

Even with the balance trimmer on the PI plates a lot of the tubes won't get close enough so it's worth the money to get a balanced one.

Henry cool comment.. I've messed around with matched tubes in the PI.. You think it makes a difference?..I only tried it once so it could of been that amp or tube.. What do you hear with the matched tubes...Curious
Tony VVT

[heisthl]

Tony - I'm just sayin' if you want the preferred 3 to 6 volt difference the balanced tube means you'll probably get there.

[talbany]

Tony - I'm just sayin' if you want the preferred 3 to 6 volt difference the balanced tube means you'll probably get there.

Cool!!

[swt]

by the way...which side gets the higher voltage...because some schems shows the neg feedb one, ans others the input one.

[CHIP]

For low tolerance MF resistors many of us here like the Vishay/Dale RN series resistors.
Military spec and they work well as plate resistors because they don't drift with age or heat.

http://www.mouser.com/catalog/catalogUSD/640/655.pdf

Thanks Tom, but I actually was refering to Henry's comment about using metal films in the PI, not just the V/D ones.

Greg

[David Root]

I like PRP 1% metal films. They are also very quiet. I have not as yet used these in V1 or V2 as plate resistors in a D-clone, I have used V/D RN65s there. I have put PRPs (1W) in a Dumbleized amp of my own design (other than the ODS circuit) in the PI as plate and other resistors.

There are two grades, GP and PR9372. GP is General Purpose, ie not designed specifically for audio frequency use, and PR9372 is the audio grade. Doesn't seem to be much difference in price, or function as far as guitar amps are concerned.

They come in 1W, 1/2W and 1/4W like the V/Ds. 1W and 1/2W are rated 500V, so they are similar in many respects to the V/Ds.

They are not color coded, you have to read them. They are red in color.

Finally they are "full size" like the V/Ds, not the newer "reduced size" stuff which open up in plate service, and USA manufactured.

[Structo]

I'll have to try carbon film on the PI plates sometime to see if I can hear a difference.
I have the RN65D's in that spot now.

[Guitarman18]

CHIP wrote:

What brands of metal film resistors are prefered? I once read to stay away from xicons.

Chip,

I asked this question about 18 months ago and at the time some senior members were anti Xicon MF's (e.g. glasman) and yet others had no problem with them (e.g. ayan), so one man's poison.....

There was a consensus that Koa Speer were quite highly regarded for their MF's, as were the NTE packaged MF's.

https://tubeamparchive.com/viewtopic.php?t=6090


Hope this helps.

Cheers,

Paul.

[kd]

Thanks for all the input. Since I'm going to order a 12ax7LPS I'll go ahead and get the balanced tube. Somehow I didn't order enough metal film resistors. Are the metal film resistors also the choice for the plates of the Dumbleator tube?

Ken