Adding the Phase Inverter Tube

I have an HD 130 410 that does not have the 12AX7 Phase Inverter. However, there is a provision on the chassis to add the socket for this tube. Two questions; first, is it worth it in terms of tone? Second, is it difficult to do?

Wow, nobody responded to your question... And it's been a couple months....
Well I think it can be done, I have messed with these amps quite a lot. First the Solid state driverboard need's to be replaced with a eyelet board and wired up appropriately of course.
I need to look at the schematic again, some of them had a whole different preamp with a transistor/diode clipping circuit, which you could get more saturation than the 12ax7 version stock, and a lot more punch from the SS driver. If it was prior to that design and just the driver circuit, then it should be a piece of cake. The idea of converting the SS distortion circuit to a Z-drive type circuit has some folks quite intrigued. If you want the classic HD-130 tone where the master volume is located between the 12ax7 stages, you get more of a classic HD-130 tone, and if you use good pedals, it's hard to beat as a Blues/Rock tone machine stock, think late 70's Tom Johnston, and all he added was a delay when I saw him and JBL's. I would not install the high voltage suppression diodes you will see on the 6ca7 plates (Pin-3 to ground) These have been known to short, taking out the output transformer, and possibly one of the reasons MM went with the SS driver. The SS driver is not really a bad circuit, it is used in many later MM amps, the RD-50 is a very popular amp with a 12ax7 Limiter circuit for the overdrive, which is in many ways similar to a Dumble ODS, and can be made to sound much like one It might be easier to mod the amp you have to sound better than you think, than to mod back to the earlier version using the 12ax7 driver, cost-wise it might cost roughly $150-plus for the socket, 12ax7, eyelet board etc. Then you will need to look at the preamp and determine if it has the transistor/diode clipping circuit. Yours may be before that and still have the earlier preamp, making it easier to mod. These can be modded to sound pretty good and not really need to use overdrive pedals. If you want to hear what a modded HD-130 (12ax7 driver version) I can email you some clips. I have done many Clapton mods to these amps, one HD-130 Bass head was converted to get tones, like "Shine on you Crazy Diamond" It will not get you the Big Muff distortion alone, but most Blues/Rock tones can be had. The model you have could be made to get more saturation if that's what you want. Most people like the 12ax7 driver beacause it has a very natural tone when working properly. If fact if you compare the schematics of the old Fender Split/Concertina? driver found in the Princeton and 5E7 Bandmaster which are some very sweet natural sounding amps and the 5E7 has been used on countless Dobbie Brother's guitar solo studio recordings, "China Groove" for exapmle. Maybe why Tom Johnston liked the HD-130 during the late 70's, with the same type driver pushing the power tubes in a sweet way like the old Fender's, a solid state preamp, that gave a decent clean boost, pushing the tubes even more than a stock Fender. So if you tend to go for Tom's vintage tone, the 12ax7 Driver is a good choice, if you want more gain, then the SS driver and preamp with transistor/diode clipping may be for you...
Email me if you have any questions.
Edgoforthsound@yahoo.com
I have a guy sending me his Solid State driver version to mod to the 12ax7 driver version.

I didn't even see this one either. Actually, I had one that had been converted to SS driver and I converted it back, with all the original pieces. Honestly, it wasn't worth the effort. I have never liked the MM distortion so it was always better to pedal distort the amp, to me. It was a pain in the ass to convert back, even with the original parts. I had the parts because I am the one who switched it to the solid state drive back in the 70's, with a kit from MM. Still have the original sheets for the conversion actually. Of course, that amp is the one that I stupidly dropped a pair of clippers into while I was working on it and it was live. 700 volts doesn't bounce very long before it took out some stuff. Never DID get around to fixing it. I also have a scan of the original clapton mods, hand written showing the changes that they did at the factory for Eric. Nothing really major actually. Personally, I'd leave it alone since it WORKS and just pedal it and use an external eq to get the right effect and tone. JMHO. Mike.

Hi Mike, so you liked the 12ax7 Driver version better is sounds. I think it has more natural warmth and rich harmonics with the Tube driver. I tried some of the Clapton mods in my personal amp, it sounds good for some things and can be tweaked to sound awesome, but I am pretty much with you, about using pedals for more variety and simplicity of use. I would add a resistor in series to tame the brite switch, or maybe some coupling caps to a better quality for more clarity etc., but These amps take overdrive pedals quite nicely, kind of like how David Gilmour uses his rig, setting the Hiwatt's or whatever, fender etc. to a full clean tone, with maybe just a hint of breakup, and using pedals to do the overdrive/distortion, it works very well. I have been able to get pretty good overdrive out of these amps with a little tweaking here and there to work better with and without pedals, depending on what the user wanted. The solid state driver sounds a bit sterile to me, except for the RD-50 with the 12ax7 Limiter circuit before the SS driver, it might be cool to add the 12ax7 Limiter circuit in place of the SS/diode clipper circuit in the HD series and get a great 12ax7 OD tone out of the HD with the SS driver. That 12ax7 Limit circuit can be tweaked to sound very good.

Gentlemen: Thanks for your advice. I think I'll keep it stock. I thought it would be a simple mod like add socket, tube and connect wires A & B. It sounds fine as it is and it does take pedals well. I'm running mine on JJ KT-77s and Celestion vintage G10 speakers.
Cheers!
Steve