bogen cha-20 hack

[DummyLoad]

well, sort'a... ok, it's mostly stock.

modified the first two stages. got rid of the gird-leak bias to V1a, added 2.2K and 2.2uF bypass. added a 680nF bypass to V1b. the phono pot became a master volume. the PI and the output stage were left alone.

i modified the PS somewhat. installed a CE dist 10-10-10-10 450V can for the screen and preamp power rails: original had a 5-5-5-5 450V.
i dropped the power rail string (R24 & values to bump up the B+ to the preamps.

B+ is high - about 475V. R31 22R 2W is not on the sams photofact - this unit had it installed and appears to be a factory mod. R32 is a mod don by me. C1 is a ARS 32uF/16uF 500V can. on a footnote, sams photofact schematic indicates B+ as 450V.

i have a pair of general electric 6V6GTA installed: surprisingly, no red-plating... overall, the amp has really good tone & the tone stack control range is decent.

original sams photofact schematic is too large to attach, so just mine with as modified, as built schematic attached.

--pete

[tubenit]

Those are nice amps!  I've done two of those as Carolina Blues Specials.  Added one more tube to have reverb & it sounded great.

with respect, Tubenit

[DummyLoad]

Those are nice amps!  I've done two of those as Carolina Blues Specials.  Added one more tube to have reverb & it sounded great.

with respect, Tubenit

this CHA-20 cleaned up fairly nice. on the other hand, i have a K10 that looks like it had a fire. charred wires, smoke stains, caps reading near short, etc. amazingly enough, the PT and OT seem to have survived: at least they test as OK. down side is i had to gut it - too many burnt wires and the output tube sockets were charred.

B+ makes me just tad nervous... 475V on mine. with 60's-70's line power it was around 455V, according to howard sams & co.

--pete

[Jack_Hester]

B+ makes me just tad nervous... 475V on mine. with 60's-70's line power it was around 455V, according to howard sams & co.

My line voltage is typically 123vAC, and I've seen the same troubling increase.  But, when I add my thermistor (120 ohms cold, rated for 3A), the line voltage drops to 118-119vAC.  Much closer to the 117vAC specs of the time period for these old amps. 

I figure the de-coupling caps have taken a beating over the years, because of this increase line voltage standard.  These last old Gibson Thor amps that I've just finished all needed new canned caps, two having the bottoms blown out.  They were rated for 500vDC, and I witnessed the 6CA7 plate voltages approaching 600vDC (without thermistor in place).  With the thermistor installed, plate voltages never got above 485vDC.  And, when the Bias current began increasing, they dropped back to 470vDC with a current of 37mA. 

So, installing a thermistor has become a must, when I go inside an amp.  Just ordered another batch from Mouser yesterday.  Ampegs got me started on thermistors, as it seemed to be their standard on early Portaflex amps. 

Jack

[tubenit]

B+ is high - about 475V

Pete, I remember one of the ones I did was 476v. My son in law has it still and 7 yrs later the JJ 6V6's work fine.

With respect, Tubenit

[tubenit]

I found a photo of my Bogen to Carolina Blues Special conversion.  I essentially just built a wood box that screwed to the sides of the Bogen chassis.  The reverb had long wires that went to a reverb tank in the speaker cab.

With respect, Tubenit

[gjesse]

I know this is an old thread, but I'm trying to follow this mod with a CHA-20 I just acquired.  It'll be my first effort in the tube amp modification realm...
So if anyone's still listening I could use some advice. 
I recapped the power section per the original specs.  It appears the power tranny is not stock, and it's producing only 310 AC volts on the secondary (measured across the center tap) with 120vac primary input, where the original schematic indicates 410 VAC is expected from the original PT.  So the B+ at the power tubes is 400 VDC where it should be 455VDC.  The output volume is VERY low, at least in part to this... there are plate and filter cap resistors that measure in spec with the ohm meter without power, but basically go open (small to no voltage drop across them) when powered.   Obviously I need to replace these pronto.
The big question...Should I replace the filter caps with larger values to boost the B+, or just start over with an in-spec transformer? 

[PRR]

Welcome.

400V on a couple 6V6 ought to be plenty loud. Leave that (non-)problem for now, and look for other possible (probable) mis-wires and wrong values.

> plate and filter cap resistors that measure in spec with the ohm meter without power, but basically go open (small to no voltage drop across them) when powered.

"Open" would mean MAXimum voltage. The most likely cause of no voltage across plate system in a tube amp is dead heater or missing cathode connection.

[DummyLoad]

you may have a weak rectifier - try replacing that.

as already stated, very low output is/are other problem/s. post a link to some high resolution photos: dropbox is a good place to start if you don't have cloud repository already.

--pete

[gjesse]

PRR, thx for the correction...I meant the opposite of open...guess I should have said the resistance drops under power.  So for instance the plates on the pre tubes are 100v or so higher than spec. Just need to replace all those resistors and go from there.  After I get them back in spec I'll report back.  Good to know I don't necessarily need to replace the tranny....
Pete, The rectifier tube is one I bought for this project and was told it tested strong.  It seems to be doing its job as far as I can tell, the vdc increases proportionally to the vac when I increase the variac.  The low Vdc on the backside (400 vs expected 455 at the 6v6 plates)seems to me to be due to the low vac on the front side coming from the tranny secondary (310 vs expected 410). 
I'll post photos once I'm back to spec on the bad resistors.
Thx for the input!

[sluckey]

there are plate and filter cap resistors that measure in spec with the ohm meter without power, but basically go open (small to no voltage drop across them) when powered.

A resistor will have no voltage dropped across it unless current is flowing through it. Are the tubes plugged in when you observed small to no voltage drop across the resistors?

[gjesse]

Good point Sluckey...no, those measurements were with only the rectifier in place. 
I put in the rest, connected the speaker, and plugged in my guitar and strummed once to verify the problem is still there, and it is...sounds great, tone controls and gain and master volume all work and sound good, just not at all loud.  Like a loud whisper maybe...

Then I let the guitar quiet down and measured again. Now I'm getting
265 vac off the secondary (should be 410)
350 vdc at the #8 pin on the rectifier (and thus on output tranny primary and the 6v6 plates) (should be 455)
207 vdc behind the plate resistors at V2 (should be 255)
168 vdc behind the plate resistors at V1 (should be 200)
The preamp plate resistors seem to be Ok now, reading 75vdc at pins 1 and 6 of V1, and 89 and 98 on the same for V2

Tracing the components, everything appears to be wired correctly...
Pete, the rectifier seems to be doing its job , but all the voltages are low, especially on the output tube plates (105 vdc too low).  Considering the low secondary vac, do you still think replacing the rectifier makes sense?  If not what should I check next? 

[DummyLoad]

pull the rectifier and measure the secondary voltage. pictures? difficult to assist if we don't have a visual.


--pete

[sluckey]

265 vac off the secondary (should be 410)

Do you have this amp plugged directly into a wall outlet? You should.

All voltages will be low if you are using a light bulb limiter.

[gjesse]

 Sluckey, no, the last measurements I reported were plugged into the variac set at 120 vac (confirmed with meter), and the rectifier tube in place.

I just plugged directly into the wall and removed the rectifier and measured 121.8 at the primary and 325 vac across each secondary lead and ground, 650 across the 2 leads. 

I put the rectifier back in and measured 245 vac at each secondary lead.
490 across the 2 leads.

All the  vdc readings were the same as when it was plugged into the variac set at 120, as reported previously.

Hopefully get some pictures up today.

[sluckey]

Sluckey, no, the last measurements I reported were plugged into the variac set at 120 vac

OK, that's fine to use a variac. I was just trying to make sure you weren't plugged into a light bulb current limiter.

[DummyLoad]

measure DCV at pin 8 of either 6V6 to ground.

tack in two 1N4007 diodes as shown in the attached schematic and pull the rectifier tube, power-up and record DCV at the specified nodes. i'm thinking you mis-wired the PS rails when you replaced the filters.

the values shown in the attached schematic in parenthesis are what you might expect based on the NL rating given for that PT. if you don't get at least that, then you probably have a wiring error, (stated again, sorry) or a defective component, or possibly a defective filter that may not have been replaced.     

--Pete


EDIT: removed bogus schematic. 

[gjesse]

Thanks for the advice Pete...I checked pins 8 to ground and they both read 16.75 v.
I'll order some diodes and make a SS rectifier to replace the tube. Apparently I could use the B+ boost it should give anyways, with this PT.  Reminder, this is a replacement PT, not stock.  I'm pretty sure it's doing what it's supposed to do, just to different specs (lower secondary voltage) than the stock Bogen one.
Thanks for the schematic, but I'm confused about what the diodes are going to the heater supply for?
If I assemble per that schematic and pull the rectifier tube, why would pin 8 of the rectifier socket, and thus any of the tube plates, have any voltage on them?  And wouldn't I melt the tube filaments connecting diodes from 300 volt secondary leads to 6v heater leads?
Quite possible I'm missing something here...steep learning curve this tube amp business....

[DummyLoad]

my apologies - in haste, i drew the  orientation of those diodes incorrectly. please see attached.


--pete

[gjesse]

Wonderful...all makes sense again...thx Pete!
I'll post results, and pix, soon

[92Volts]

I was going to suggest the power transformer was generally overloaded (by a short somewhere) until I realized it's a replacement, which causes the low voltage.

However, low heater voltage would cause low current (and low volume) so you should check that too. Just to be sure the power transformer is fully working.

Plate resistors won't change resistance under load (at least that would be unusual) but a bad tube, too large a cathode resistor, or bad connection between plate/cathode resistors could be a problem. No grid reference to ground could also prevent proper bossing.

[gjesse]

92V, I'm getting 6.5v on the heaters so I think alls well there.
I'll doublecheck all the cathode/plate resistor connections.

[gjesse]

The top shots are as I found it.  I added 1/4" input and speaker out, tore out the phono stage and made the phono pot the master, like Pete's schematic. 
The interior shots are post mod. 
Don't dog me too much in the solder skills or component placement...just trying to get it to make sound with volume to it for now...


https://www.dropbox.com/sh/zjur3ywgb99gmcd/AAB9rVcKIU0JwBln-4i_7XSla?dl=0

[gjesse]

If someone had tried to test this with tubes in but no speaker load, they might have blown up the output tubes and the output transformer, right? What would be the symptom of a damaged OT?
I've replaced the output tubes to no effect. 
Could the OT be bad?  I'm measuring AC voltage and current across the OT primary, but VERY little of both on the 8 ohm speaker tap, with speaker attached of course.

[SoundmasterG]

If someone had tried to test this with tubes in but no speaker load, they might have blown up the output tubes and the output transformer, right? What would be the symptom of a damaged OT?
I've replaced the output tubes to no effect. 
Could the OT be bad?  I'm measuring AC voltage and current across the OT primary, but VERY little of both on the 8 ohm speaker tap, with speaker attached of course.

If there is no signal going into the amp, and the amp itself isn't oscillating on it's own, then it would be fine with no load. I've done this many times myself without any damage or issues whatsoever, and was told it is fine to do so when I was first getting started in tube electronics 20+ years ago by a long time EE that I know. There is often much more damage with old gear from the capacitors being dried up and power being applied without the device being brought up to proper AC voltage on a variac and current limiter.


One symptom of a damaged OT is a strangled and very low and distorted output signal. Since a transformer is an impedance matching device, there will be a ratio between primary and secondary windings. You could disconnect it from the amp, and run a signal from a signal generator through it on the primary and measure what you get on the secondary. Sometimes you can run the signal the other way...secondary to primary, but you have to be careful about the voltage and current or you can damage it with that test. Once you have a result, you can see what the ratio between primary and secondary are. If it is a known transformer with published specs, then you can compare to what the specs are. Likewise if you know a particular tap is say 8 ohms, then you can figure out what the actual ratio is. If a transformer has been damaged, then these numbers will likely be off from what they should be. That said, OT failures are a much more rare occurrence then many techs who don't know better assume.


Are you measuring any DC voltage on the OT primary windings? If so, what is that?


Greg

[92Volts]

I've killed an output transformer before 
The speaker came unconnected as I was playing. I double-check those connections now!
If I had not been playing, SoundMasterG is right that it likely wouldn't have been a problem.

The symptoms were sound fading in and out, sometimes very low-- but not consistent.
If you're hearing that, a bench test might not reveal a short which comes into play when there are large voltage swings.

If it's behaving consistently, but wrong, a bench test is more likely to demonstrate the problem.

Impedance ratios are the square of voltage ratios. This is useful since you're more likely to know impedance specs (at least ballpark) than the voltage/winding ratio for an output transformer.
So if we have a 5k ohm plate load to 8 ohm secondary... 5000/8=625 impedance ratio.
sqrt(625)=25 is the voltage ratio.
If we apply 6.3v AC to the 8 ohm tap we expect 6.3*25=157v AC across the primaries.

[DummyLoad]

please see attached... can't tell if that speaker jack is insulated from the chassis - swap them anyway in case the jack can come into contact with the chassis + not wise to have the signal on the sleeve.


--pete

[gjesse]

PETE!! You're the man!!  I was playing my chassis the entire time it appears...
Was going too fast when I wired the output jack and looked right past it when tracing my work.  Total newbie move...
This thing jams!  I was expecting it to be more of a small practice amp in volume, but even with low voltage off the PT secondary It's loud enough to wake the kids and wife upstairs.  Gonna wring it out in the morning.
THANKS EVERYONE who responded to help me get this figured out, especially you Pete. 
I learned a ton doing this first simple amp mod.  I feel like I could easily build one from scratch now.  Maybe I will, but first to perfect this mod...
Can't wait til morning!

[DummyLoad]

YW. enjoy. i gave mine away to a young musician here in austin. i nursed his twin long for a few years, and he was using it as a practice amp at home: a twin is hardly a decent home practice amp, so i gave him the CHA-20. the bogen with a delay pedal and he's in heaven.


--pete

[gjesse]

Sounds as good as it looks!