77 Boogie 100 Watt

I have a 1977 boogie 100 watt head that is original. Still operating but while replacing the original pre-amp tubes to try to fix an occasional "breaking up" problem, I discovered the V1 "fetron". After doing some research, I think I found everything available on the fetron. Then talked to Mesa Boogie and got a bit of information from them on what needs to be done to replace with 12AX7. Although they have responded, they really haven't explained it really well yet ... exactly what needs to be done if I have to replace the fetron. I've got another information request for further clarification into them now.

My question: Anybody out there know exactly what needs to be done to replace fetron with 12AX7? Heard they had something on the Mesa website but it was nothing near a complete answer. In fact, I had the same paperwork in my files at home.

Also: What does it sound like if you make the adjustment? I've been pretty satisified with this amp for 30 years. However, since they stopped
using fetrons about a year after mine was made ... makes me wonder ... is it better without the fetron? Hard to imagine it being better?

Not sure that I'll ever have to change it ... it may not be the cause of my occasional problem ... but since they don't make fetrons anymore ... I'd like to know all I can about it.

This might give you some help.

http://www.mesaboogie.com/manuals/Mark%20I%20Maint-Repair.pdf#search='fetron'

http://home.earthlink.net/~ayan/history.htm

Thanks. I found both of these documents ... both helpful. Actually had a 30 year old paper copy of the first ... it says you can just replace the fetron with 12AX7... it does run but it's just not a good deal. Something is missing in the performance. I got a MK1 & MK2 schematic (both with Fetron notes) from Mesa Boogie ... gave me some clues but it's not enough for me yet. The fact that they made specific changes leads me to believe the performance of the 12AX7 needs these changes. Where I stand right now with Mesa boogie is as follows:

Clear that I should:
- Add 1.5k 1.2 watt resisitors to pins 3&8 (cathodes) of V1
- Replace resistors on pins 1&6 (plates) of V1 with 100k 1/2 watt resistors

Things that I think may be so ... but were not specifically answered:
- Use 60mf/350v capacitor to ground per MK1 schematic? If so, any resistor in parallel (ie 470k on MK2 schematic)?
- Use 33k-15k resisitors in series per MK1 schematic?

I really want to be sure, if I have to adjust this amp ... it's gotta be right! I've heard Mesa Boogie takes care of their old customers - so I'm hoping they'll continue to guide me to a properly adjusted amp. It is an awesome product!!! 30 year satisfied customer!!

The IA schematic shows the plate resistors are V1 A/B 100K and instead of cathode resistors there are 1N5303's instead of typical resistors. The cathode decoupling caps are already on the V1 A/B. It seems it is as easy as removing the 1N5303's and adding resistors in parallel on the cathodes and leaving the resistors at 100K 1/2 watt resistors.


Where does Mesa state the plate voltage to the V1 should be?
With the TRI009 is's only 115V. It's 186/195 on V2.
If V1 is being fed by the "D" tap at 260 and being brought down to
186/195 by the plate load resistors then why even use a 15-33K. The
MK IA Schem calls for these only if the Fetron is used. It's 225V after the 15-33K and I beleive the 60uf/300V is used to stabilize the Fetron.
The original MK I is fed by the "D" tap and has no 60uf/350V cap to V1.
It's filtered in the power supply. I would remove the cap, diodes (1N5303) and the 15-33K to get it to a stock MK I.

Any thoughts?




http://www.tubefreak.com/mk1.gif

So:
- Replace in5303 diodes with 1.5k 1.2 watt resisitors to pins 3&8 (cathodes) of V1 ... I was reading the note as "in addition to" but it very well could mean "instead of". Place them in parallel with the caps.
- Replace resistors on pins 1&6 (plates) of V1 with 100k 1/2 watt resistors ... as on the original drawing
- Mesa did not mention the voltage ... but my original schematic showed "D" tap at 260v.
- I think you are right ... remove the 60uf/330v & 15-33k they seem to be drawn in for the fetron
- is there an original drawing w/o the fetron available somewhere? The drawing I had that came with the amp did not have the 15-33k or 60uf but it did have the IN5303 diodes instead of 1.5k resistors ... so it appears to be a "mix" of the two drawings. It did not say Mark 1 ... since there was no such thing at that time. I could scan that schematic if it would help.

Thanks for the assistance ... I think this is helping. Are you a technician? I am not.

You have a PM.

Well it's fixed and it worked great ... with a little help from Boogiebabies on this website. I got a bit of help from Mesa Boogie but if it had not been for cruising on this website ... probably would not have fixed it.

It sounds great with an off-the-shelf 12AX7 ... better than before!

Sweet !!!!

I am very glad that you like it. I bet you are thinking it was easy now.


Ed

Yep ... took about 30 minutes. But it's easier when you really know what to do.

kdove said:

So:
- Replace in5303 diodes with 1.5k 1.2 watt resisitors to pins 3&8 (cathodes) of V1 ... I was reading the note as "in addition to" but it very well could mean "instead of". Place them in parallel with the caps.
- Replace resistors on pins 1&6 (plates) of V1 with 100k 1/2 watt resistors ... as on the original drawing
- Mesa did not mention the voltage ... but my original schematic showed "D" tap at 260v.
- I think you are right ... remove the 60uf/330v & 15-33k they seem to be drawn in for the fetron
- is there an original drawing w/o the fetron available somewhere? The drawing I had that came with the amp did not have the 15-33k or 60uf but it did have the IN5303 diodes instead of 1.5k resistors ... so it appears to be a "mix" of the two drawings. It did not say Mark 1 ... since there was no such thing at that time. I could scan that schematic if it would help.

Thanks for the assistance ... I think this is helping. Are you a technician? I am not.

Click to expand...

Hey there! It sounds like my 1978 Mark I needs the exact same treatment as yours, is this list exactly what you ended up doing, including getting rid of the big 60uF (350v in my case)? And where are located the 15-33k you said to remove?

Mine does not have 1n5303 but two TCR5302 instead, but I guess they serve the same diode purpose anyway?

Thanks,

Frederik