Looking for Bogen CHB 20A Schematic (Bogen CHB20A Project)

[sluckey]

Those .1 and .22 caps are not electrolytics. The .1 cathode cap in the Ampeg can be a low voltage cap. 25 volts is enough. 630 volts is way overkill but will work also. I used a .1@100v just because I had one on hand.

It's rare to see an electrolytic that is less than 1µF.

[PRR]

> I can't find an electrolytic cap with .1 value

Where does it say "electrolytic"??

You *can* use electrolytics for cathode caps. Above a few uFd, the cost/bulk of film-caps "forces" you to e-caps.

And I bet I could find 0.1u e-cap if I hunted.

But here, the cost of a film-cap won't break you. Lowest voltage you can find, since there's never even 3V here.

And it is a tweak. Different amps, speakers, and players may want more or less or none. It sure will work without it. 0.1u here adds a little zing.

[Platefire]

Well I went through my NOS cap inventory and did find four .1 caps. Three\100nF/35V and one 400VV as shown in picture. Also I've attached a pic of the .22. Don't guess it is electrolytic! It just looks like it is because it's so large. I guess the .1 I found will work, just need to locate a .22/400.

Body of the .1/35V is about 1/4"L X 1/8" Dia. The 400V one is about 1 1/4" Long X 3/8" Dia. You can barley read the small ones they are so small. I did check them with my Fluke and they measured from 95nF to 100nF.

I guess on the bias, I could go ahead and change the 25/25V electrolytic with 22/25V have on hand and see if that picks up my bias voltage. Now that I know the .22 is not electrolytic, I may can find one now! Platefire 

[Paul1453]

Rather large aren't they?

[Platefire]

YEAH, vintage was all big. I've got a lot of NOS resistors and a lot of them are about the size of Mallory 150 caps---they look more like caps than resistors. Some of the Maroon colored ones measure right on the money of their value---very precise. Platefire 

[sluckey]

One of those little .1 caps looks perfect for a cathode bypass. I bet there is nothing wrong with that .22 in your amp. I'd leave it alone.

[DummyLoad]

sorry plate, should have stated this a bit more clearlyr:


C15 - .22uF film cap.


C16 & C17 - 2 x 25uF electrolytic caps.


  


use a 400V for the .22uF and 25V parts for the electrolytic caps.

[Platefire]

I should have caught that on the .22. I guess I forgot on all vintage P to P, everything is super sized.

I have scrapped up enough parts to build all of sluckey's  preamp with the exception of the 470pf but I do have a 500pf and also the .0047 but I do have a .0033 and a .0058 cap. I think I will use the .0033 because it measured 4nF on my Fluke and the .0047=4.7nF, so that pretty close. Platefire

[DummyLoad]

I should have caught that on the .22. I guess I forgot on all vintage P to P, everything is super sized.

I have scrapped up enough parts to build all of sluckey's  preamp with the exception of the 470pf but I do have a 500pf and also the .0047 but I do have a .0033 and a .0058 cap. I think I will use the .0033 because it measured 4nF on my Fluke and the .0047=4.7nF, so that pretty close. Platefire

you could just use the 5.8nF for now. 


--pete

[Platefire]

pete----that's what I call splitting hairs. So what is your rational for the .0058 over the .0033----larger is less bright/more bottom? just wondering? 

[DummyLoad]

pete----that's what I call splitting hairs. So what is your rational for the .0058 over the .0033----larger is less bright/more bottom? just wondering? 

not much difference - a flatter response curve with 4.7n. the 5.8n has a slight dip at 150Hz. the 3.3n has a larg(ER)bump at 250Hz.   


--pete

[Platefire]

OK, Thanks! While we are talking difference in values, one I failed to mention on the bass pot, the cap from the input term to output term is a .001. I didn't find a .001 but did find a .0015. My thought was, this couldn't make much difference(.5nF larger) being so close in value but being a hack of sorts, I end up second guessing myself Platefire

[DummyLoad]

OK, Thanks! While we are talking difference in values, one I failed to mention on the bass pot, the cap from the input term to output term is a .001. I didn't find a .001 but did find a .0015. My thought was, this couldn't make much difference(.5nF larger) being so close in value but being a hack of sorts, I end up second guessing myself Platefire

it's fine. slight bump at 400Hz center when controls are set to flat.


--pete

[Platefire]

Well new preamp and TS coming up, guess I'm good to go to "TINKER CITY"

[Platefire]

I had a rude awakening last night! After a session with my amp of installing parts I was watching TV relaxing and thinking about what I had just installed and then----WOW! I realized nobody installs a 47K cathode resistor!! I had missed the decimal between the 4 & 7 and mindlessly installed a 47K. So thanks to my big NOS carbon comp resistor inventory, I was able to quickly locate a 4.7K. That will be the first thing I correct on my next session. Platefire 

[sluckey]

he, he, he. I've fixed stuff in my dreams too! Sometimes I even remember the fix the next day!   

[shooter]

nobody installs a 47K cathode resistor

I downloaded Sluckey's preamp, looked at it twice, and was gonna post "why 47K"  thanks platefire !

[Platefire]

Ye Haw! Got the wiring complete on the new preamp today about 2:00 PM. I fired it up shortly thereafter on a light bulb limiter, there was no smoke, the amp is working but I little noisy or maybe a lot noisy. I fired it up again without the light bulb limiter and the voltages looked pretty good but the noise was bad as ever. The louder I turned it up the noisier it got. I couldn't understand that because I wired up the grid with shielded cable all the way up to the the PI.

So I started rechecking my wiring wondering what I did wrong? Then I decided to try a different 12AX7 in V1 and that did it----that fixed it!!! Now it's nice and quiet like it should be.

At that point I started to do some sound testing:

1-I can turn the MV up 100% and just use my pre-gain as volume and it will stay clean up to about 7 before it starts breaking up. The response at 7 is a pretty loud clean volume. Way more clean headroom than the previous arrangement. I think the clean volume is loud enough to use anywhere I would ever play.  You can also roll back the MV and get more touch sensitive feel. It's not a high gain monster by any stretch of the imagination but you can dial in some nice crunch settings using the MV.

2-The tone sounded best with the setting set flat with the humbucker guitar I was using. The tone stack seemed to do a good job of dialing it in a lot darker or brighter but set flat was just about the right balance for me.

3-The bias voltage is still running about -13.75V where it should be -16. I did change out the two 25/25 bias caps out with two 22/25 caps. I guess I need to go ahead and replace that .22/400V cap next feeding the bias supply and see what that does. On the 12AX7, V1A was 166V and V1B was 182V plate voltage. The power node feeding the preamp is 254.2V

First impression is I really like the amp. Operational noise is very minimum, Clean headroom is way improved like I wanted, the tone stack seems to be doing a good job and the pre & master volumes seem to be interacting well with each other. I would think if your looking to get the most clean headroom out of a CHB20A, just put this preamp in front of the existing power amp. You can also get some nice feel and crunch playing around with the volumes. At this point I'm very satisfied. 

Thanks very much sluckey for providing plans for the preamp that hit the mark and everybody's input. I will leave the amp opened up until I get the bias running cooler and sure everything is OK. Now let me go fire it up again and jam a little>>>>>:>O Platefire 

[Willabe]

Now let me go fire it up again and jam a little>>>>>:>O Platefire

       

[Platefire]

It's Weird, with that tone stack I can turn the treble from completely flat at 12:00 to boosting the treble to about 3:00 and it's like kicking a light overdrive in---it really punches it. On the other hand you can cut treble and bass, and it cuts overall gain. Guess in a way you could use the tone stack as kind of sensitivity control to some extent. This Baxandall type  tone stack seems to have more effect on volume than Fender/Marshall/vox type tone stacks. Platefire 

[Willabe]

It's Weird, with that tone stack I can turn the treble from completely flat at 12:00 to boosting the treble to about 3:00 and it's like kicking a light overdrive in---it really punches it. On the other hand you can cut treble and bass, and it cuts overall gain. Guess in a way you could use the tone stack as kind of sensitivity control to some extent. This Baxandall type tone stack seems to have more effect on volume than Fender/Marshall/vox type tone stacks. Platefire

Sure.   

The more signal you let through the more it pushes the tubes.

[DummyLoad]

glad you kicked some life back into that old thing.


pics? sound bits?     



--pete

[sluckey]

I'm enjoying that clean sounding Ampeg preamp in my latest conversion. Nice change from the typical Fender or Marshall. That tone stack has a different feel to it but it does a good job whether you're using a Strat, Tele, 335, or Les Paul.

[Platefire]

Yelp, I'm enjoying everything about my latest version with the exception of my bias needs to be brought under control. These power tubes are way too expensive to be burnt up.

The cleans are great which is my main sound but the distortion when brought up, is really rich and good harmonics. I will repeat, it's 100% better than before and is totally usable in a live playing situation--which for me means good loud cleans.

I haven't got any sound clips but do have some pics. IMHO old style P to P is not a pretty site to behold especially in my case;>{ Platefire

[sluckey]

IMHO old style P to P is not a pretty site to behold especially in my case;>{ Platefire

That's exactly how it should look! 

[DummyLoad]

IMHO old style P to P is not a pretty site to behold especially in my case;>{ Platefire

That's exactly how it should look! 

+1


+ nice work.


--pete

[tubeswell]

question on the schematic sluckey posted, the TS shows a 120k/270k between the bass and treble wipers.  What's their job?  I'm drawing up an amp that uses essentially the same type TS, without those R's

Put a 250k to 500k Pot in there wired as a variable resistor, and you have a variable slope control, or more simply, short the resistor with a switch


Here's a schematic I've built into the last three amps I've done now using this type of tone stack with various goosed-up bits

[Platefire]

I have considered drawing a schematic of this project. I searched out the SCH files on this site for a Bogen CHB20A but nothing. Before I launch into putting this together---has anybody drawn up a SCH file for this amp?

  Platefire

BTW-if I wanted to install a 10K bias pot into this bias circuit, would replacing R26 with the bias pot be the way to do it? Schematic attached!

[sluckey]

BTW-if I wanted to install a 10K bias pot into this bias circuit, would replacing R26 with the bias pot be the way to do it? Schematic attached!

I don't think that will do it. Before you get too far, measure some voltages. I bet the voltage on either side of R26 is about the same. What did you actually measure? Is the goal to run the tubes cooler?

[Ed_Chambley]

R29 looks like a candidate for a bias pot.  Here a 50K pot looks like it would work to me, but you may need to lower R23 if you are not even at -16.

I would just use the Marshall style as the Bogen is similar anyway.

With higher line voltage you would think the bias voltage would be more.

[Ed_Chambley]

question on the schematic sluckey posted, the TS shows a 120k/270k between the bass and treble wipers.  What's their job?  I'm drawing up an amp that uses essentially the same type TS, without those R's

Put a 250k to 500k Pot in there wired as a variable resistor, and you have a variable slope control, or more simply, short the resistor with a switch


Here's a schematic I've built into the last three amps I've done now using this type of tone stack with various goosed-up bits

Tubeswell, on your schematic, are those 1Meg pots on your "slope switch".  Cool  idea.

Is your raw switch a pot too, or just 1meg to ground?

[DummyLoad]

try changing R24 from 12K to 22K. 


--pete

[Platefire]

Ok, I tried DummyLoad's suggestion replacing R24 12K with a 22K resistor. The bias is now reading -15.87V.
I'd say that's close enough to 16 for me. Good call DummyLoad!!!! At least I can jam on it now without having to keep looking at the tubes to see if they are red plateing.

sluckey, yes I was wanting the tubes to run cooler. They were very hot to the touch. However they never red plated because I was keeping a close eye on them. I thought if I had a bias pot I could dial the -16V in. R26 now reads -15.87V on both ends. This is with the amp loaded, it's -16.5V unloaded.

Thanks, Platefire   

[DummyLoad]

glad she's  humming along now... it would be cool to hear it as well... no pressure!   

--pete

[Platefire]

Is there any real simple way to record a clip--like on my smartphone I do have a camcorder on my PC, but I tried to convert a vidio/recording I made with it where I could post it and never could get there. I do have a Tascam 8 track digital recorder I haven't used in a long time---I would have to get the manual out to do anything with that. I'm always getting request for sound clips and I guess it would be good to know! Platefire

[DummyLoad]

if you have a smartphone those usually work ok if you stand a few feet away.  post it on youtube and post that link here.

--pete

[tubeswell]

Tubeswell, on your schematic, are those 1Meg pots on your "slope switch".  Cool  idea.

Is your raw switch a pot too, or just 1meg to ground?

Hi Ed


Those are just 1M resistors which can be shorted with switches - takes less space than pots. But if you want, you can use 1M pots wired as variable resistors in either/both locations

[Platefire]

Question on the bias circuit? C16 .22/450V reads 206VDC on PT red secondary side and 14.15VDC on the bias circuit side. So if a coupling cap is to block DC, is this cap leaking? This red secondary from PT is a un-rectified ACV from PT---so where is the DC coming from? the diodes?

I haven't changed out C16 with new yet, so I guess what I'm trying to figure out is, is this 14.16VDC prior to my bias diode negatively effecting the bias circuit???

I'm just trying to tie up any loose ends on this amp before I put it amp back together. The bias voltage is still right at -16V like it should be. I can't help but wonder, if I change C16 with new, would the bias voltage change again? Platefire 

[sluckey]

I don't think there is any reason to change that .22 cap. Your voltmeter is not telling you the whole story. Trying to figure out what's happening in that bias circuit, especially with that .22µF cap, is gonna be difficult with just a voltmeter. A scope makes it more obvious. Take a look at the attached file for an visual explanation...

[Platefire]

Yeah, that is a good illustration of how the negative Voltage is achieved. I know I build these type circuits by schematic and never give much thought of how it actually works. Thanks, sluckey. I'm satisfied that everything is well enough to button up and just enjoy a while. I'm going to eventually take it to church and use is with the Praise band.

Also this is the first amp that I've used shielded cable throughout the preamp section all the way to the PI and this amp is gloriously quiet! I love it! tubenit has preached shielded cable for years and this is the first time I've actually done it.     

Gosh! the 6GW8/ECL86 are way more expensive than I ever dreamed. I thought they would be high, but they are even more than I thought--I guess because they are all NOS or used, no new. That's even more incentive to get your bias right Platefire 

[sluckey]

Do you want to make the bias adjustable? Or convert to cathode bias?

[PRR]

> A scope makes it more obvious.

Agree.

It appears he "has a 'scope". Platefire's image looks like a circuit simulator. Such things usually have Waveform Display ('scope).

I'd bet the waveform at R23 R24 is a big AC wave, slightly offset to the positive side, but with peaks that run way negative. D3 catches those negative peaks for our use.

If the secondary is 142VAC, and the other side is at AC ground (C2), then the R23 R24 point eyeballs at less than 1/3rd of 142V, say 30V. Oh, but on negative peaks there is added loading from R25, so maybe 20V. The 1.414.. factor does not apply because the peak-catcher's source (R23 R24) is not low-low impedance relative to its load (R25 plus what comes after). We'd ballpark say 18VDC. The simulated 16.4V is in the ballpark; closer than we deserve after saying C15+R23/R24 was 1/3.

[Platefire]

No, I'll leave it as is for now. If I have problems some time in the future with the 6GW8 tubes I may rip out the power amp wiring and install a socket for a separate 12A___PI tube and rewire for EL84's. I've got plenty of extra EL84's. There is plenty of room in this chassis for expansion, so that would be doable if need be. At that time it may be a good time to install cathode bias. The 6GW8's are just going to get higher and higher unless someone starts manufacturing a new version, but I doubt it because I don't think there is enough demand to cause it to happen. Platefire

[DummyLoad]

>> It appears he "has a 'scope". Platefire's image looks like a circuit simulator. Such things usually have Waveform Display ('scope).

that's DL's (moi) simulation image.  it's multisim by national instruments. yes, it has a scope. didn't think what was required was that in-depth of an analysis, so just posted DMM results.

--pete

[DummyLoad]

No, I'll leave it as is for now. If I have problems some time in the future with the 6GW8 tubes I may rip out the power amp wiring and install a socket for a separate 12A___PI tube and rewire for EL84's. I've got plenty of extra EL84's. There is plenty of room in this chassis for expansion, so that would be doable if need be. At that time it may be a good time to install cathode bias. The 6GW8's are just going to get higher and higher unless someone starts manufacturing a new version, but I doubt it because I don't think there is enough demand to cause it to happen. Platefire

are EL84 going to handle 400V of B+?

i'd leave it be.

--pete

[Platefire]

To answer your question DummyLoad, I guess I thought the ECL86 was basically an EL84 with one 12AX7 gain stage trowed in. I think I'm right on the 12AX7 triode but the pentode stage is capable of handling a lot higher voltage than an EL84--looking at the specs.   

I don't think no one really indicated for sure where the bias pot should go except Ed said at R29. So yeah, since we got this far, it would be nice to know where it should go and how to wire it up. You guys have to forgive me, I get amp battle fatigue after I've fooled with an amp so long---but for sure a bias pot is really needed to preserve these rare tubes.

Plus there is the issue of how many mA should be safely running with the 412VDC plate voltage? I'm looking at my RCA data sheet on this tube for maximum ratings. Plate supply voltage triode=550, pentode=550. Plate voltage triode-300, pentode=300. Also cathode current--triode=4 ma, pentode=55ma.

I'm not understanding the 300 plate voltage above---what is that? My plate voltage is 412V

So this is maximum ratings. I guess what I really need to determine is what ma I could safely run the tube and still get a good sound. The bias adjustment would need to get on both side of the happy ma setting. I could always use the shunt method to find what ma it's actually running. Platefire 

[DummyLoad]

550V is the Vao and Vg2 rating at zero current flow or cut-off. the max Va and Vg2  is 300V when in conduction. bogen is pushing those little tubes pretty hard to get 20W. they're running in AB. it is essential you keep the plate dissipation at idle at the recommended range. at 400V with -16V on g1 they're idling at about 17mA or about 75% of max plate dissipation. you could idle them colder but you do so at risk of adding cross-over distortion.


another other option is drop in the 12AX7 and to use NOS 7189A - definitely not low cost and are scarce as well; the upside is they have a Va rating of 400V and a 13.2W Pdiss rating: the bias supply would only require minor tweaking, if any at all.


the all out medieval option is add the 12AX7 hole and punch out the 9pin power tube holes to accommodate octal sockets and run a pair of 6V6. also, bias supply mod required.

try running the 6GK6 at 400V? they're a 7189 with a different pin-out.

--pete

[Platefire]

pete---thanks for the profile on Bogens operation(hot) of the 6GW8 and possible alternative options. I haven't completely given up on obtaining some extra 6GW8's. Looking at e-bay and talking to friends with NOS stashes.
Usually on e-bay if you look intently enough, you can shake down a good deal eventually. I'm always scanning ebay for stuff, so that would be no chore. Got some on my watch list. 

I did put the amp back in its case yesterday. Last check of voltage was 412 plate voltage and -16.5 bias.
Lowest previous reading I've got since changing the bias resistor to 22K was 400 plate voltage and -15.87 bias. I'm not sure rather it is a fluctuation in household AC voltage or a power tube fluctuating is causing the variation or not, but that seems to be the range it's presently operating in--highest and lowest. The two working tubes I have in there are definitely running cooler than they were, so that bias adjustment helped. One tube is one of the original Bogen tubes and the other is a NOS Lendel I just recently installed when I discovered one of the Bogen tubes was bad. So I presently have no backup 6GW8's---but looking! Platefire

[sluckey]

AES sells 6GW8s for $40.

[PRR]

> I thought the ECL86 was basically an EL84 with one 12AX7

By the plate dissipation, and heater demand, it may be like 3/4 of an EL84.

Pd is 9W, not 12W.

A '84 plus a 'AX7 would eat 0.91 Amps at 6.3V. '84 only sucks 0.7 Amps, less than a '84 alone.

Another point DL made: these tubes were originally rated to run at 300V nominal max (330V if you actually measure every amp you make, which is true in DIY). The later 7189 is the same tube but was was rated 400V. With the collapse of 7189 production, a lot of people tried EL84 in 7189 amps, some lived some died, and "generally" now most guitar-market "EL84s" will survive 400V. However vintage ECL86 were not made for such use/abuse, and I think it is risky.

Using ECL86, clearly you are not building a stadium-blaster. 20 Watt versus 10 Watt is not going to make-or-break the amp sound, and 20W@400V might break these rare tubes. _I_ would drop the B+ down close to 300V.