CJ

Peavey Series 300 Bass Amp
« on: April 01, 2019, 04:19:41 PM »
looks like an early model, no schemo online so we emailed Peavey for one,

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
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CJ

Re: Peavey Series 300 Bass Amp
« Reply #1 on: April 01, 2019, 04:24:03 PM »
Six power transistors, RCA NPN, probably 3055 nomenclature,

rails at about 36 volts each, we replaced the caps with some Marantz pulls which tested good, so that upped us from  4500 to 9000 uf,   why put 50 bucks worth of caps in this antique?



If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #2 on: April 01, 2019, 04:25:36 PM »
preamp is all discrete , there are opamp schemo's online but this apparently predates those versions,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #3 on: April 01, 2019, 04:32:46 PM »
we tested all the RCA 40409/10 driver/heatsink transistor modules and one had a low hfe compared to the others so we are splicing in a TIP 50 upgrade onto two of the driver heatsinks , hfe is about the same, maybe a tad bit lower but we now have a 500 volt rating at 1 amp which should withstand any speaker back lash,

this amp had a broken lead on the air inductor used on the output snubber which cooked the 22 ohm bypass resistor so we fixed that up and put some hot glue on there to prevent bass vibration,



If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #4 on: April 01, 2019, 04:44:12 PM »
snapped lead on the coil>

you can see the flame broiled resistor above the coil, medium well.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #5 on: April 02, 2019, 03:24:00 PM »
Peavey does not have this exact amp in the files,

but they sent me this>

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #6 on: April 02, 2019, 03:24:57 PM »
this looks pretty early>



If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #7 on: April 02, 2019, 03:25:37 PM »
supply>

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #8 on: April 02, 2019, 03:26:22 PM »
pdf>
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #9 on: April 05, 2019, 07:01:11 PM »
and the 24 V regulator with the open collector zener effect on Q4,

stuck a HV diode in there which has the twice V drop in case you were scratching your nogin,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


PRR

Re: Peavey Series 300 Bass Amp
« Reply #10 on: April 05, 2019, 09:38:19 PM »
>   most of the transistors have no idle current until signal is applied

Q 10 11 12 13 are indeed idled "off".

Q8 Q9 can put significant power to the load before Q10-13 cut-in; however their bias looks weak (714mV for two diodes?). Also the diodes should point downward. The diode above that is suspiciously drawn-- it smells more like a third bias diode than an insignificant fudge on the 1.8k resistor. (Three diodes IS the right value for biasing Q6 Q7 Q8.)

Q11 base comes from Q9 emitter, not collector.

> strange they they use emitter resistors in the collector also on the bottom half pwr transistors, this causes an offset and there is no pot to dial it out,

That's not causing your offset. (The overall NFB should dial it away.)

The collector resistors are "needed" for the protection circuit current-sensing.

Your drawing shows NO base resistor on Q1. That won't work. It works, so there must be a resistor.

Cover-up the taped-in 1.8k, which looks wrong to me. Assume Q1 has 100k base to ground, reflecting the 100K DC path to Q2 base. If Vbe and hFE were matched, the drops across 100K Q1 Q2 100K would mirror-image giving zero offset. The match won't be perfect, but <50mV routinely happens, and that is fine for a hundreds-watt woofer. 120mV is very suspicious.

*With* the 1.8k and two 1K, it would balance if Q1 base resistor were 4k-5k, but the 0.22uFd input cap says that is unlikely. Also I see no reason to take DC NFB from Q8 emitter resistor.

Much of this seems to be drawing error and decades of part-drift. If it plays well, close the hood and call the client. But if it keeps coming back, you may need to get the schematic righter to know what it is supposed to do.
« Last Edit: April 06, 2019, 03:39:35 PM by PRR »

JohnRoberts

Re: Peavey Series 300 Bass Amp
« Reply #11 on: April 06, 2019, 09:46:06 AM »
we reversed this rev pwr board,  most of the transistors have no idle current until signal is applied,

this amp distorts at about half volume so i bet they reduced the gain on subsequent models,

strange they they use emitter resistors in the collector also on the bottom half pwr transistors, this causes an offset and there is no pot to dial it out,
Those are for the current limiting in that direction. I don't see how they could affect DC offset.

BTW there appears to be a R missing from base of Q1 to ground.

JR
Quote
nice amp if you are going for that Sly Stone fuzz bass on Dance to the Music but do not want to use a stomp box,  :D
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Gus

Re: Peavey Series 300 Bass Amp
« Reply #12 on: April 06, 2019, 10:34:29 AM »
CJ

Did you trace the preamp?
I find some of the older solid state preamp schematics  interesting to look at.

CJ

Re: Peavey Series 300 Bass Amp
« Reply #13 on: April 08, 2019, 12:03:44 AM »
hopefully the amp will come back and i can trace the preamp!

thanks PRR for the analysis!   :D
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #14 on: April 08, 2019, 12:18:00 AM »
well we lucked out tonight and the rev we were working on popped up!  :D

no i get to see how bad i screwed up,   :o
« Last Edit: April 08, 2019, 02:23:21 AM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Re: Peavey Series 300 Bass Amp
« Reply #15 on: April 08, 2019, 02:10:10 AM »
we re-typed the numerology on this thing and gave it a pdf label,

full props to PRR for predicting the 1.8K - 18 K boondoggle, the diode direction, the Q11-Q9 thing,  :D

1971 design, so this is probably the oldest Peavey that has come in the shop, Peavey started in 65.

there are a few differences in the front end, like the input network and the 680 instead of 1K in the diff amp. and we had left off a few caps that are probably to keep things stable,  looks like the output section is indeed biased off,

we stuck a TIP50 in there for an RCA 40409 pre-driver  which comes right after the diff pair , which might explain the cold bias on the other pre drivers,

also, the 1K-1K divider for the NFB seems to have a different takeoff point on one of the resistors, or i might have traced it wrong,

this amp could use some gain reduction,



« Last Edit: April 08, 2019, 02:46:40 AM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

JohnRoberts

Re: Peavey Series 300 Bass Amp
« Reply #16 on: April 08, 2019, 10:45:14 AM »
we re-typed the numerology on this thing and gave it a pdf label,

full props to PRR for predicting the 1.8K - 18 K boondoggle, the diode direction, the Q11-Q9 thing,  :D

1971 design, so this is probably the oldest Peavey that has come in the shop, Peavey started in 65.
The designed by HDP Is Hartley himself, drawn by HTC is Hollis T Calvert, another long time employee. That was apparently before Jack Sondermeyer took over the amp designing.
Quote
there are a few differences in the front end, like the input network and the 680 instead of 1K in the diff amp. and we had left off a few caps that are probably to keep things stable,
and a DC path to ground
Quote
looks like the output section is indeed biased off,
weird, the driver stage has class A/B bias but the final outputs nada... Legend has it Hartley had trouble keeping them from melting down and spent so much time on the phone talking to Sondermeyer at RCA, he talked him into joining him
Quote
we stuck a TIP50 in there for an RCA 40409 pre-driver  which comes right after the diff pair , which might explain the cold bias on the other pre drivers,
I see three diode drops and three Vbe junctions so class A depends on how hard you hit those diodes with bias current. It looks like a double diode was attached to the heatsink so he was trying to thermal compensate the class A current.

At low power the drivers were driving the load through the .68 emitter resistors,  10 mA later the first output turns on, at 20mA or so the other two...

My suspicion he was struggling with reliability.
Quote
also, the 1K-1K divider for the NFB seems to have a different takeoff point on one of the resistors, or i might have traced it wrong,

this amp could use some gain reduction,
High closed loop gain attenuates the NF making it easier to stabilize... The quasi complementary output topology adds lag in one polarity of swing that requires a little more compensation, but quasi complementary was widely used by early solid state amp designers out of necessity (poor selection of robust PNP outputs).

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

CJ

Re: Peavey Series 300 Bass Amp
« Reply #17 on: April 08, 2019, 05:58:51 PM »
Wow, thanks a lot John!  :D
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

PRR

Re: Peavey Series 300 Bass Amp
« Reply #18 on: April 09, 2019, 12:53:06 AM »
> At low power the drivers were driving the load through the .68 emitter resistors,  10 mA later the first output turns on, at 20mA or so the other two...

I read it:

Q6 Q7 (my numbering) idle at 150mA, a decent value and crossover will be low. When Q6 or Q7 load current rises to 0.9A, Q8 Q10 or Q9 Q11 start to turn-on. This will be around 3 Watts. At max output (10A) we have like 2.5A in Q6 or Q7, 3.7A in Q8 Q10 or Q9 Q11. Six transistors share the load.

Idling Q891011 below cut-off is an old trick. I'm surprised you had not seen it. Thermal stability was not well understood, and the few who had a trick did not publish. Hard to find the happy spot between crappy sound and thermal runaway. However a stone-cold bias hard-B stage never runs-away at idle. A small power stage on a large heatsink does not run-away. Q6 Q7 is the small amp. Q891011 is the hard-B amp.

Q891011 are so cut-off that he had to take DC NFB at Q6 Q7; it probably was unsteady taking DC NFB from the nominal output. Or maybe he only feared that.

There is current limiting but no voltage sensing, thus no VA or SOA protection. This may have been OK with the fat-crystal devices of the late 1960s. Current limiting is just adequate for 4 Ohms *resistor* and may ding reactive loads.

Q3 has nominal current-limiting. (When the output stage melts-down short, the last step is little Q3 trying to pull the whole load, another 3 bucks shot.)

Yes, CJ, leave the gain AS-IS. NFB stability was another new thing at the time. (Has been around since Watt but specifics of power transistor best-practice were hazy.) The gain stage is fast but the output stages are a cascade of slow and slower parts. The gain is not high for a power amp. If it is high for the combined amplifier, put a pad before the power amp.

That's a sneaky ~~15KHz 2-pole low-pass in front of Q1. I bet if you test-bench it with higher frequencies the output stage melts (cross conduction).

Noted voltage at Q5 base is wrong polarity.

PRR

Re: Peavey Series 300 Bass Amp
« Reply #19 on: April 09, 2019, 01:22:37 AM »
me> If Vbe and hFE were matched, the drops across 100K Q1 Q2 100K would mirror-image giving zero offset.

Now that we have more-right values, we see that Hartley knew his stuff. Q1 base is fed with 10k+4.7k+4.7k or 19.4k. Q2 base feeds from ((1k||1k)+18k+2.7k)||100k or 17.5k. And 19.4k==17.5k for all practical purpose, certainly in Meridian. Assume hFE is 150 and emitter current is 1mA each. Base current is 0.067mA each. The resistor difference makes 13mV offset. In that day, you would plan for 10mV difference of Vbe. Feedback factor at DC is >1000. So the idle DC output offset should be held to around 25mV, which is entirely zero in context of a big stage speaker which will survive 35V chunks of bass.

The 18k tail resistor is not a perfect current source. The audio gain is set large for several reasons. One is to keep the input voltage, and common-mode voltage in the input pair, small. This reduces THD.

The input pair balance is better than was often seen in the day. It does rely on V+ being about 65 Vbe, but normal wall-voltage variations won't shift it much off-balance. Later studies have shown point-oh-oh level improvement in even-order THD if the input pair balance is *exact*. But that needs more parts (typically a current mirror) and a little 2nd is not offensive in a stage amp. Maybe not measurable since this is only 4 stages of current gain (until Q891011 cut-in) so total THD will be in the point-something range, not point-oh. (Recall that the reference amp would be 6V6 or 6L6 rising quickly to 1% and 2% before clipping.)


 

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