PI in a 100W HRM

[mdroberts1243]

Does the PI have to change for a 100W?

Fender circuits seemed to favour 12AT7 and a change in the cathode resistor of the PI for four output tubes (although I've seen the standard 12AX7 PI in earlier circuits).

There was a great thread on the BluesMaster and PI changes, but it didn't touch on the difference between driving two output tubes vs. four.

[Darkbluemurder]

I posted the same question regarding the tube.

As far as I know Fender used a 470 ohms cathode resistor and a 12AT7 tube in almost all blackface amps, regardless of whether it had 2 or 4 output tubes. Since the 470 ohms resistor is not connected to the output tubes but only to the PI input it does not matter how many tubes the PI drives.

HAD used 820 ohms in the cathode but a 12AX7. The resistor lowers the gain somewhat compared to the 470 ohms but the tube has more gain.

[mdroberts1243]

Thanks.

I am concerned about the values of coupling caps... maybe you have to go back up to the 0.1uf for the bass response?

Also what about grid stop resistors? Four of them? In the 5F8-A Fender used two and two of the tubes didn't have grid-stop resistors.

BTW, after re-reading the Blues Master thread I caught reference by AmpDoc1 to his excellent spreadsheet (new to me)... http://ampgarage.com/forum/viewtopic.ph ... preadsheet

Not clear which, if any, of these were 100W, but worth digging into.

[jackdc100]

I have a dumb question partly because I am new to the site. What is a HRM? I'm assuming it is one of the Dumble models, but I don't recall hearing of a Dumble model with those initials. Thanks,
-Jack

[novosibir]

Jack, HRM = HotRodMod, some also say Hot Rubber Mod or whatelse. Anyway, they all mean the same - the additional 'marshall-ish' Tone Stack after the overdrive section, which is fix adjusted by 3 trim pots.

Larry

[odourboy]

Mark hasn't received a reply to his original quesiton yet and I'm curious as well...

so... BTT

[FUCHSAUDIO]

I've always seen 12AX7 drivers, regardless of 50 or 100-W in Dumbles.
The only changes were the FB resistor from the output transformer. Maybe an occasional adjustment in the plate load resistor values, but always 12AX7's.

[chris_sanford]

Mark hasn't received a reply to his original quesiton yet and I'm curious as well...

so... BTT

The answer is 'no' (the pi doesn't need to change). The 101 schematics are actually based on 100 watt examples.

chris

[mdroberts1243]

Thanks Guys!

Spent some time with the schematics archives on the weekend and pretty much convinced myself that the PI wouldn't have to change.

So... I plan to use something like the Marshall JCM800 Lead Series preamp and four-tube power amplifier as the basis of my Funk-Chassis-Project, with the PI values tweeked as per AmpDoc1's spreadsheet (BluesMaster column)... the JCM800 already has the 100k NFB and .022 couplers, etc.