Amp Mods Harmful

Circuits

I own and operate an Audio Tech Service business in Boston and
I'm the same person that posted about the scarcity of 7027A's. I DID
call "Vudu Electronics" but they never had the courtisy or professional
ethic to return my inquiry.

Reading some recent posts on the subject, I thought I'd share
some experience I have had here. Pardon my spelling.

Once upon a time a customer came in with an old Ampeg; I think it
may have been a Reverb Rocket. Whatever it was, it had 7591 output tubes
and this guy wanted it converted to 6V6's. I looked up both tube types
and advised him against it. As I best remember it, not only was the pin
layout totally different, the key problem was that of transconductence.
I beleive the transconductence was almost half (or double) from one tube
type to the other. I suggested that we change the output transformer to
a commonly available type such as Fender if we wanted to do the job right
BUT I still cautioned the customer that there was no guarantee that the
amp would "sound" exactly the same. as it had.

The customer INSISTED that it would work. HE HAD READ THE MOD IN
THE "GROOVE TUBE" BOOK! He even produced his copy. OK, so I did the mod
exactly as the book said; I rewired the sockets, changed screen grid
resistors, and trimmed the bias just past cross-over distortion AND
BEHOLD: HALF POWER! CRAPPY DISTORTION! PISSED OFF CUSTOMER!

When this guy cried afterwards, I DID NOT return his money. I
gave him what he asked for. I told him to take his complaints to the
Groove Tube people. THEIR job is to sell people like HIM "groove" tubes,
and I am not responsible for their design mods. THANK YOU.

People come to me all the time begging me to "mod" thier amps,
and I simple will NOT do it. In fact, I DO spend a lot of time DE-
modding classic amps for people who have learned better. You simply
would not BELIEVE the kind of HACK Mickey-Mouse crap that crosses my
bench. It never leaves that way; I just won't allow it.

You know something folks? It never fails to amaze me when some
egotistic little poindexter (no reference to Groove Tube) thinks he's
going to "re-design" or major modify an amp in his cellar workshop.

I used to work in the R&D department of what was once THE leading
pro audio equipment manufacturer in the world. In fact, I co-designed
(among other things) the audio compressor which became the world standard
for live sound and recording. Please beleive me when I tell you that a
LOT of time, money, and experience went into that design. And you
know what? Somehow, I suspect the same is true about most of these
classic tube amp designs too!

I no longer design audio equipment; I repair it! I don't fool
myself about R&D because I've been there. My shop and I am NOT going to
compete with an engineering department that has an R&D budget of 6 figures!

Anyways, my contacts at Ampeg tell me there is NO mod presently
available for the 7591's that "sounds" ANYTHING like the original design.
How COULD there be? 7591's are so superior a tube to 6V6's that its like
trying to replace a V-8 engine with a 4-cylinder one :)

They also tell me that the 7027's should be reolaced with 6550's,
NOT EL-34's or 6CA7's. as others may tell you. I tend to trust what the
manufacturer's R&D and/or Service Departments have learned and researched
FOR me.

I guess what I'm trying to share with all of youis to watch out for
the ego hacks and all the cheap advice. I've been in this business a LONG
time and one thing I've learned is that PART of being "the best" is to know
when to let go of my own ego and call upon OTHERS' exertise.

Bottom line is that I don't have all the answers. If anyone else
has some ideas; post them. Then let's run them by Shawn Cahill and the
rest of the boys at Ampeg. OK?

Jack Donahue
Audio Tech Services
Allston, Massachusetts
(617) 254-1600

Index:

AB crossover	Amp Classes Defined	Amp Classes Explained	Amp Mods Harmful
Bass Cutoff Frequencies	Bias adjust safeguard	Bias Cathode Resistors	Bias Shift
Biasing methods	Biasing	Channel Switching LDRs	Choke Input Filter
Circuit Memory	circuits	Drop B+	Eyelet board
Fender Response Curves	FET source follower	Filament Cathode Ground	Fixed to Cathode Bias
FM Transmitter Voltages	Grid Blocking	Identify Power Trans Leads	Load Impedance
LV No PC boards	Output impedances	PC Boards	PCB amp problems
Phase Inverter Resistors	Pilot Light Tone	Rectifier voltage drops	Resistor Design
Resistor Max Voltage	Resistor noise	Reverb Tank Types	Reverb tanks
Screen Grids	Standby Switch Ratings	Standby switches	TAME Digest
Tone Circuits	Transformer Design Book	Transformer Info	Transformer Placement
Zener Voltage Drop