Bogen CHB14A project

[RogerWilco99]

I bought a Bogen CHB14A off eBay some years ago and it's been sitting on the shelf waiting until I had time to do something with it.

Well, now I have the time and now that I have had time to really look at the amp, I don't really like what I see.

The tube complement in this amp is an 6V4/EZ80 rectifier, a 6AU6A/EF94 preamp tube for the mic input and a pair of ECL86/6GW8 tubes.

The 6GW8/ECL86 choice for the power amp with their own built in driver was an economical idea I suppose but it's not exactly common, I don't think. At least, not in the world of guitar amps.

I've been looking at the condition (and doing some measuring of disconnected components) and nearly all of the passivle components need to be replaced as the caps are all mostly either shorted or open circuits, the four way can cap is no good with two shorted caps and two that are WAY off (rated at 10mf and measure to be close to 2mf on my capacitance meter). The resistors have drifted all over the place and one of the larger ones is a bit crumbly. So, it's all new passive stuff needed.

The heaters are all working so at least the 6.3v secondary on the PT works. I haven't checked the HT secondary to see what voltage it's putting out. And I haven't much idea what current this PT is capable of. Although I suspect I can back into that by adding up the current needs of the 6AU6 and the 6GW8s and use that figure as a ballpark.

I've no idea if the output tranny works at all. The amp would make no sounds with a speaker and mc hooked up when I first started fiddling with it. But that could just be because the circuits are dead. The power up test with no tubes didn't yield any sparks or a tripped breaker (the thing doesn't have a fuse). I'm getting 6.4V on the heater tap and 364V on the HT tap unloaded.

I have a complete new set of tubes for this. So, I could just rebuild it in its "standard" configuration and hope the output tranny works. I've already decided to put an IEC power entry jack on it. And add a fuse to it (this one does not have a fuse even though the manual says it does, right below he power cord -- but that area on the chassis is blank with no hole).

What I have rolling around in my head (yeah, it's lonely in there) is to convert this amp to use a pair of EL84s and to replace the 6AU6A pentode and its 7 pin socket with more common noval tubes, a 12AT7 as a PI for the EL84s and add a 12AX7 as a preamp tube. I'm also considering changing the EZ80 to an EZ81 but the EZ80 probably can handle all the current that the PT is capable of (another way to tell its current output, I suppose) so that change probably wouldn't net me anything even though I have one on head (well, three actually, but that's another story).

Anyway, I'm of two minds here. I could just rebuild the amp in its original configuration or convert it to a more "conventional" design and build it like a Marshall 20 watt. I'd have to change the pots on the front and lose the bass and treble switches in favor of pots. But I like having a three way tone stack. The cover would not fit, if I did this without dirlling holes for the pots.

I guess it would be more visually appealing to rebuild the amp as original, while leaving out the AUX inputs (and the mixing pot for them) and converting the output to use a cliff jack (or jacks) and changing the mic input to a couple of phono plugs for high and low impedance, too (probably also a cliff jacks).

Either way, I'm going to have to put in all new components. It will cost about he same either way since I already have all the tubes for either build (including the matched 12AT7). I will have to put in two new tube sockets if I change over to the EL84s since I'd need another pre-amp tube and the 6AU6A/EF94 is a septal (is that the correct term for a seven pin socket?) socket and would have to be replace with a noval for the 12AT7.

So, which way would you guys go? Go with the unusual that might sound like garbage and possibly have to deal with a lot of tweaking to get a good tone? Or go with a known good design and sound that "might" require some new iron?

I'm leaning toward the Marshall 20 watt design right now (I'm a Marshall fan, anyway -- most of my store bought amps [and gigging amps] have been Marshalls). But it's possible that the Bogen design might yield a sound that's good but "different".

Cheers,
Bruce

[gldtp99]

Interesting plans for the CHB14A----i have a CHB14A around here somewhere that i haven't re-done yet--- i just sorted thru a stack of CHB20A's looking for the 14 but it must be over at my workshop.
I have redone several CHB20A's and a Bogen MU-10---- all use the ECL86/6GW8 output/PI tube---- the MU-10 is cathode biased and uses an EZ81 rec tube and a 6EU7 preamp tube instead of the CHB14A's 6AU6A---- MU-10 has the slide cut switches for Bass and Treb instead of a tone stack---- the CHB20A's have Bax-type tonestacks and are fixed bias with larger iron--- 6EU7 in preamp, too.
I removed the single multi-cap can from the MU-10 and replaced it with two F&T 16/16uF@450V cans--- one mounted in the orig can slot and the other next to it---- i put a 1/4" input jack on the front panel and removed the Channel vol pot, Aux pot--- leaving the Master vol as the only Vol pot---i also changed some tone caps to better voice the amp for guitar----- this amp has a very aggressive tone and gain attack (full on Med-hi gain--- not modern metal but screams hard rock) but can clean up with the guitar pot rolled off--- it compares very favorably with a 14/7 watt (switchable pentode/triode) EL84 cathode biased, JCM 800 2204-style head i built (soundclip here---- click on "Mule 14/7 JCM 800"-- http://www.soundclick.com/bands/defa...?bandID=741589 ). The Bogens don't sound exactly like the Marshall 14/7 clone or even the real Marshall model 1917 20 watt PA head i serviced for a friend recently---- but they do sound good, just different.
I wouldn't worry too much about keeping the CHB14A stock ---- if you want the Marshall circuit with the TMB tonestack then go ahead and build it---- these aren't ever going to be collector items and lots of folks re-do amps like this.
I'd measure resistance on the OT primaries to try and see if it was completely blown before i spent a lot of time/money on the re-build----- measure from ECL86 pentode section plate to other ECL86 pentode section plate---- and then from each plate to the CT---- the CT-to-plate should be @half the plate-to-plate reading---- they won't be exactly the same because one winding has more wire (same # of turns but wound outside the first winding) but should give a good idea whether it's good or bad.
Wouldn't hurt to measure resistance of the secondaries while you're at it just to see if the 4/8/16 ohm taps are intact----- then see if there is any resistance between primaries and secondaries--- anything but an open circuit is a problem here.
It sounds like your CHB14A is in good hands---let us know what it comes out like...................gldtp99

[RogerWilco99]

I disconnected the OT on this and did the continuity and resistance checks. Everything appears to be fine. There are no clear shorts. And each of the impedance taps has a different impedance. So, I put it back in and soldered it back up.

Just for grins, I hooked up a speaker, plugged in a mic and turned the amp on. After giving it time for the heaters to come up, I eased up on the mic and master volumes until I started to get some microphonic sounds of me handling the mic. Lo and behold, it lives. The mic worked fine and the sound was fairly clear coming out of speaker.

Next test was to plug in the little Squier Junior Strat I've got and hit the strings a few times. (I can't play anymore because of my neurological condition) It didn't sound very good and there was a lot of variability to the sound (louder, softer, etc) without touching the controls.

Anyway, after having that little revelation, I know the iron is probably okay as is apparently the tube complement that's in there (I put in all new tubes when I got it) is working okay. When I first got it, I found an ECL86 in the rectifier socket (No wonder the guy never got it to work...).

There must have been a bad solder joint in the OT tranny wiring. Because it works, now and didn't before (no sound at all before).

I also calculated the current requirements for a Marshall 20W type circuit (2x12AX7 and 2xEL84s) and I don't think this PT has the HT current or the heater current for the conversion.

So, I'm going to rebuild this with the current tube complement, put normal phono plugs as the speaker hookups and switch it over to a Marshall style tone stack. I'll probably end up giving this away to a friend or relative when I'm done. If none of them want it, I'll probably just stick it back up on eBay and let someone who really wants a good tube amp but doesn't have a lot of scratch have it. I doubt it would go for more than $200. I'll much more than that in it in time and materials. But I will have had a lot of fun working on it and tweaking the circuits.

If anyone has any tips for re-voicing this amp for guitar, let me know. It got pretty loud with the Tube 10 speaker. When I cut in the bass it got pretty throaty. Even with that little speaker. The treble cut doesn't sound too good. I'll try the Marshall style tone stack first to see how that sounds.

The mic gain and master volume pots need to be replaced. They're quite loose internally and even though they don't scratch, they just have a noisy quality (volume jumps up and down as you turn each one not in keeping with the position of the knob). This might be because of the crud in the rest of the circuit. But I think I'd prefer all new pots so I know they're tight, reliable and haven't drifted.

I wonder, though, if replacing that 6AU6A with a 12AX7 might be a better idea. That way I can put the gain control between the two stages and the tone stack after the second stage with the master volume. Or should I just stick with the 6AU6A? The voltages and current should not be a problem. The 6AU6A is a pentode tube but can be run as a single triode. I just thnk perhaps a dual triode would work/sound better. It would also be a preamp I'm more familiar with.

Oddly enough, switching out the 6AU6A for a 12AX7 would make all the tubes noval instead of the three novals and a septal (is that the correct term?).

I would also like to put some sort of retainer on the ECL86z. I believe retainers for EL84s would work. I think the tubes get too hot for the shield type retainers you usually see on 12AX7 tubes but I'm not at all sure of that.

Here's the CHB-14A schematic again just so anyone who wants to can look at the original schematic and make suggestions and maybe tell me whether swapping the 6AU6A for a 12AX7 is a worthwhile or sensible swap.

Oh, the voltages for the PT under load are 306-0-306/613, so says my Fluke 115 DMM (red, red-yellow, red). The heater voltage under load is 6.4v (green,green). I didn't try the amp-meter to look at the current. But the PT probably puts out somewhere in the neighborhood of 90ma on the HT secondary and I guess probably 2A on the heater secondary.

If it's not obvious, the HT is center tapped and heater is not. You can't use a rectifier tube without a center tap can you? I think I read that somewhere. I don't recall if it was in the Weber book or maybe one of Morgan Jones' books.

By the way, has anyone here used or can they recommend the TubeCad program or the SE amp program from the same folks?

[gldtp99]

It Lives !!! That's good news---- i don't see any problem with converting to a 12ax7 pre tube and a tone stack-type preamp---i build lots of amps besides converting old PA heads to guitar use and i prefer to use 12ax7's as much as i can---more common and they sound good.
As far as tweaking it for guitar use, the Marshall-type preamp should take care of that---- Usually old PA heads with stock circuits need brightening up for guitar use and some tweaking of the tone stack (if there is one).
The schematic doesn't seem to match the amp you describe---- your amp has a tube rectifier with a center tapped HT secondary---- the schematic shows a silicon diode x 2 voltage doubler power supply typical of other Bogen amps with silicon diode rectifiers.
Maybe this is a difference between CHB14 vs. CHB14A---- anyway, your amp doesn't match the schematic and probably has a different PT---- check the part # on your PT against the one listed on the schematic--- i'm thinking they're not the same PT.
I have lots of 1M Audio long-shaft pots from Bogen CHB amps --- used but good---- PM me your address and i'll send one (or two) if you need them..........................gldtp99

[RogerWilco99]

I checked the part numbers I could verify and nothing with a part number matches. Not even the chassis. You are correct about this being a CHB14.

I neglected to notice that it says right on the chassis "MODEL CHB14 Series [B-82]". So, it's not a CHB14A. That's why the part numbers don't match. I must have looked at that a hundred times without noticing the missing "A". I'm looking for a CHB14 schematic/manual, now.

But the schematic is largely correct, at least. I have looked over the innards enough to be able to say with some certainty that the passive components and wiring are as shown as are all the pots, switches, inputs and outputs. And the voltages that I've measured fall fairly closely to those on the schematic in the PS.

As for the pots, thank you for the kind offer. But I have a fairly large supply of Marshall pots (I had a JCM800 2203 for about 22 years), knobs and various other bits and bobs. I think I'll probably use those instead of the Bogen pots.

When I first got the amp, I noticed that the front ECL86 was shorter than the one behind it. Upon further inspection, I noticed that it was an EZ80, not an ECL86. When I looked at the tube in the EZ80 socket, it had an ECL86 in it. So, I swapped those before ever powering it on. Two of the four tubes say "Bogen" on them. So, they're probably original. I've put in all new tubes, now anyway.

I'm going to swap the 6AU6A for a 12AX7. It is indeed easier to find and the power requirements are about the same. This way, I get two stages, to support the tone stack as well as a gain control leaving the "master" volume where it is.

There was an old Marshall amp (the 10 watt "1930") that used ECL86 tubes and an ECC83. I have the schematic for that. It doesn't have anything I can recognize as a tone stack. Of course, the schematic is quite fuzzy. So most of the writing is hard to read. Not to mention a bit odd looking.

I have attached the 1930 JMP schematic that I have. It's the only one I've been able to find. Also attached is the schematic of another JMP where I got the idea of two 12AX7s and a pair of EL84s, the JMP 20W PA Lead & Bass amp.

[gldtp99]

A CHB14--- that explains the power supply non-match to the schematic---- the model 1917 20 watter is a great sounding circuit----my friend's 1917came out sounding very good with new filter caps and careful re-tube.
I've never heard the model 1930 w/ECL86's but, if i remember correctly, i've read that it wasn't one of the greatest sounding Marshalls ever built.
Another option would be to build a Fender BF-type preamp using one 12ax7 and the classic BF Fender tone stack that's between the two 12ax7 triodes.
Such as: http://www.schematicheaven.com/fende...b763_schem.pdf
Build the Normal channel but be sure to use a voltage divider where the existing "channel mixer" 220k resistors are on the schematic (keep the series 220k but run another 220k to ground----simulating the un-used other channel's signal "bleed"). And you could keep the existing ECL86 PI/Output Pentode that's already built in the amp
This would give a more General Purpose amp tone--- it might need to be goosed by an OD pedal to really scream but it might be cool the way it is-----just a thought---- build whatever circuit you'd like to-................gldtp99

[RogerWilco99]

A new direction

I've decided to go off in a completely different direction with the CHB14 amp. I'm going to:

• Gut the Bogen chassis completely, sand and paint gloss red
• Clean up and re-install iron (no paint)
• Install cliff jacks on the rear
• Install two cliff input jacks on the front center
• Install IEC power inlet (socket)
• Install fuse holder in front panel
• Install new pots on the front for Tone, Gain and Volume
• Install new On/Off toggle switches for power and standby
• Install octal socket and 5AR4 rectifier tube to replace the EZ80
• Install 5v2a Rectifier tube heater PT for the 5AR4
• Bought Matchless Spitfire turret board
• Bought components to load circuit board and pots
• Bought three new Alpha 24mm 1Ma pots
• Install components onto Spitfire board and install into chassis
• Install new ceramic noval sockets with tube retainers
• Bought Tung-Sol 12AX7 preamp tube
• Install noval socket with shield for the preamp 12AX7
• Bought matched triode Tung-Sol 12AX7
• Install another noval socket with shield for the matched (12AX7) PI
• Bought matched TAD EL84-STR tubes
• Install matched EL84 tubes
• Wire it all up in phases and test
• Get new front panel made for new functions
• Have old cover stripped, primed and painted gloss black

When it's all said and done I'll basically have a Matchless Spitfire and I think it should turn out to be a pretty good guitar amp.

Total investment so far, about $320.

Yeah it's a lot more expensive than my first plan. But I think I'll have a better amp with tubes that won't become scarce anytime soon.

I'll also have several ECL86 tubes to sell (6 or 8), some of which are new (including an NIB Telefunken still in the box and the plastic wrapper and an NIB Sylvania).

I'll put those tubes up for sale when I'm done.