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ProReverb1
Junior tube assistant
Posts: 1
(4/22/04 7:50 am)
Need some answers for a Proreverb
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High guys
I'm doing a complete overhaul on a 68' SF Proreverb,still need to replace some resistors to finish.The only schematic i can find that matches this is a AB668,there is also 1069 & 165 that are close also.The chart on the inside of cab is basically gone! My amp has a Philips 5as4a rectifier tube,bought a 5u4g(Sovtek) which is the same thing,that's in there now.Tried a gz34(ab668 shows this) in it but it glowed a alot brighter but it seemed to sound better.My power tubes are RCA blackplates 6l6gc's,the rest are telefunken ecc81's or 83's.Speakers are Celestion/Rola G12-65'S at 15 ohms.Just a note- i just wanted to replace power cord to 3 prong & i've ended up doing a rebuild.It's been really interesting to find out
what makes an amp sound great!
Questions
-which tube is the proper one or should i use?
-on bias board it has a 1k resistor,should it be replaced with a 470 ohm 3w(what does this resistor actually do)
-On bias pot should i replace 15k with 27k or 24k resistor?
-still have to check voltages and set bias,but those questions
are for another time.
Any thoughts or answers would be great,thanks
bluesbear
Hey get your own solder
Posts: 365
(4/22/04 9:29 am)
DON"T TURN IT ON TILL YOU READ THIS!
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Everything I can find shows the Pro Reverb with a 4 ohm speaker load. Unless yours has a replacement tranny, your amp is at risk. You have the speakers wired in series. Assuming the Rolas are 8 ohm, they need to be wired in parallel. Then they'll read 3 - 3.5 ohms (that's 4 ohms impedence).
Once the speakers are wired correctly, a GZ34 should be fine (rebias).
Good luck!
Dave
ProReverb1
Junior tube assistant
Posts: 2
(4/22/04 10:32 am)
It's been turned on
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Just to let you know this amp was packed away for 19 yrs.It was bought second hand like this.Yes it's been turned on and played with 5as4 and 5u4g rect tube in it.Here's the link for schematic ab668,it shows 2 x 16 ohm speakers-www.kbapps.com/audio/sche...ab668.html .Can give you transformer number if like to check.
Thanks for reply bluesbear
EDIT: I fixed the link.
Ken
Edited by: GroundhogKen at: 4/22/04 12:15 pm
bnwitt
Senior tube assistant
Posts: 339
(4/22/04 11:36 am)
Re: It's been turned on
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Have you looked at
www.schematicheaven.com GroundhogKen
Forum Moderator
Posts: 2352
(4/22/04 12:40 pm)
Re: It's been turned on
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That's strange.
The AB668 circuit clearly states two 16 ohm speakers wired in parallel for 8 ohms. All the other Pro Reverbs that I'm familiar with have two 8 ohms in parallel for 4 ohms.
All these circuits call out the same 125A6A output transformer.
That AB668 circuit is pretty nasty looking. 47K PI plate caps. Suppression caps on the 6L6 grids. Combination cathode/fixed bias. I wonder how it sounds.
Ken
Tiny Daddy
I will work on all amps
Posts: 627
(4/22/04 2:10 pm)
Re: It's been turned on
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47K and 2000p will give serious frequency rolloff and phase shift. Then there's the .001uF coupling cap - this needs Blackface mods!
PHATamps
I only work on tube amps
Posts: 441
(4/22/04 5:09 pm)
Re: It's been turned on
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Yeah, and hybrid fixed/cathode bias, too! Hmmm, AB668 -- June '68 -- after the AB165 Bassman, this must be the second amp the CBS engineers f***ed up when they started dicking with everything.
Sounds like an excellent candidate for a Hoffman AB763 conversion to me!
ProReverb1
Junior tube assistant
Posts: 3
(4/22/04 11:55 pm)
Sounds like it's a winner!
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here's some info for ya
- power transformer- 022798
-transformer -022699
-output transformer -22848
-reverb choke -022921
Heres something things i've done already
-3 prong power cord added
-changing filter caps & resistors
-replaced bias cap to 47/100v
-replaced 470 ohm and 1.5 resistors on power tubes also cut off the 2000p, on pin 8 already had wire from pin 8 soldered to chassis,got rid of cap from pin 8 to pin 8 also
-basically everything on circuit board is almost replaced
-changed to 100k plate load resistors at phase inverter and added the 2 220k resistors that go to the bias pot,did the changes at the bias pot switching wires etc.
-switched heater wires so there both pin 2 & 7,already has 100r's at lamp
If anyone can tell me how to add pics to my posts it would be a lot easier to show you guys.There's alot more that i can add to this but thats for another post,this typing is giving me a headache.Ahh, it's time for a cold one!
Tiny Daddy
I will work on all amps
Posts: 630
(4/23/04 7:58 am)
Re: Sounds like it's a winner!
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Also change the .001 coupling cap to the phase inverter. Use .01 or .022uF.
EL34
I got your parts right here.
Posts: 2280
(4/26/04 10:34 am)
Re: Sounds like it's a winner!
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See the for Fender conversions.
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