Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on February 27, 2011, 04:35:34 pm
-
I thought I remember seeing schematics on this site for a lot of Bogan CHB series amps but I just looked and didn't turn up nothing. Anyway I was given a Bogen CHB 20A amp today and was wanting to see a schemtic to see what my options were in converting it to something. With only one preamp tube and two power tubes, it's got me puzzeled on how the amp operated on so few tubes. Platefire
-
I'm Sorry, I jumped the gun! I found it under PA Amps and see that the ECL86 is a dual preamp and power tube--thus the shortage of tubes in this amp. Should be interesting! Platefire
-
Look at Geezer's Little Wing.
-
I'm Sorry, I jumped the gun! I found it under PA Amps and see that the ECL86 is a dual preamp and power tube--thus the shortage of tubes in this amp. Should be interesting! Platefire
6gw8 - nice! i posted an amp built using those and a 12BZ7 - called the goliad - it's very similar to the hoffman 18W stout. definitely a cool tube.
--DL
-
Har! Like old times agian. Taking an old junked amp and trying to bring it back to life again. I first hooked it up to a test speaker and plugged my guitar in aux input---nothing through the speaker! I took the tubes out and checked voltages and it's getting good heater and DC voltage all accross the circuit. I cleaned the tube pins, sockets and pots with contact cleaner, put the tubes back in and now getting sound through the speaker--so I got a working amp--well the PT and OT is working anyway. So now that I know I got some working major components, now I can plan an amp :grin: Boy I love it!
-
I got other things I need to be doing but for some reason I been foolen with this amp. This morning I cut out the aux input wiring, removed the aux input vol pot, moved the mic vol pot/with wiring over to the right in the aux pot hole and installed an input jack in the mic pot hole. I cut the sheilded mic input wire to the threaded jack in the back and wired it to the new 1/4" jack. I decieded I just wanted to hear the existing circuit as is. I hooked it up to a 1-10 cab and fired it up. I've always heard how some players liked the existing Bogen circuit as is but I never gave them a chance and always gutted them to build something new.
You know I'm kinda impressed with what I'm hearing. The thing is dead quiet even with the old circuitry.
The tone stack even sounds like it's doing a pretty decent job---but I've been testing with a tele---the tru tone stack test will be with a strat or les paul. So I'm pleasently surprised that it is sounding so good with so little work done--it has to be to good to be true. Better go do some real work done before the wife begins to get on my case. :laugh: besides I'm suppose to be working on my TE-Deluxe!
-
IF you do a major conversion ........ here is one to consider. The 6GW8 and 6BM8 have some similarities. You could eliminate the PPIMV if you wanted since it would not be a loud amp to begin with. And you could use a fixed value for the mid pot if needed.
with respect, Tubenit
-
Schematic for a possible conversion here
http://photobucket.com/images/Bogen+CHB20A+Schematic/
Kagliostro
-
Thanks Tubnit and Kagliostro
I saved the schematics on my computer. I had already downloaded the LWR by DeGeezer which I know tubnit worked on too. The 56T looks highly related to the LWR. I've also been interested in a Marshall 18 Watter for a long time. I appreciate your help and looking at what options I have on this thing.
Right now I'm still recovering from the shock of my first impression of the stock Bogen circuit. I reworked a few PA heads since I started working on amps and in every case so for I've never liked the
stock tone stack and response but this one is appealing! I will have to work my way through this one before I can move along with any serious modding. I have been studying the Bogen schematic and it's hard for me to tell what's going on in the power section---I guess it's PP but hard for me to understand. If you can tell me what's going on with that Bogen power section, I would appreciate any help.
I am considering some slight changes to the stock circuit and would like your input of what you think. I have a poorly marked up scan of the schematic if you can read my Hen scractching. What I'm thinking about is rewiring to move the location of the tone stack between the two preamp gain stages and moving the MV to right before the PI that I try to show on the mark up. Let me know what you think? Platefire
-
The power amp is a classic fixed bias push/pull amp driven by a paraphase inverter. There's a global NFB loop. The power supply uses a voltage doubler for the B+.
-
Thanks Sluckey
Can you explain global NFB Loop?
-
Local NFB is negative feedback within a single gain stage and only affects the gain of that single stage. Global NFB usually encompasses several gain stages. In this case, the negative feedback source is the OT secondary. The feedback signal wraps (loops) all the way around to the first half of the paraphase inverter, a more global circuit. The global negative feedback circuit reduces the gain of the PI and the PA, tightens up the sound, improves fidelity, and decreases distortion.
-
I've been very familiar with local NFB for a long time but this is my first encounter with global that I'm aware of :huh:. That expains a lot of the extra wiring I couldn't understand. Platefire
-
Got more questions.
1-In the preamp stages for the Aux and mic channel there is a 470K Resistor in parallel with a 100(pf?) cap. Is this a high pass filter to increase highs and reduce gain or what is the purpose? I did away with all this on the Aux input including that cap and resistor because I didn't need that imput and did away with it to make room for the 1/4" guitar(mic) input on front panel. So I guess what I'm getting at, if I wire the tone stack the location where the parallel res/cap is at in the mic channel, do I need to keep this or do away with it or keep it inplace to retain the existing integrity of the amp.
2-Also If I re-wire the Master Volume right before the phase inverter--I'm thinking I need to do away with the 100pf cap to ground and seperate 1 meg R to ground. I guess what I'm wanting to do is retain the existing intergrity of amp but move the master volume to where it will be more effective to work as a conventional MV operation. I guess bottom line is I can rewire--and if I don't like it put it back like it was??
I know most of you don't agree with the wisdom of preserving the integrity of an old PA head original circuit but I tell you there is something here that I'm really liking and I don't want to destroy it by making quick surgical extractions without knowing why and what for. Platefire
-
Gave it a pretty good road test with my band and she held up pretty good. Power range about like a tweed deluxe. Had it running though my home made 1-10 cab with a 20 watt Eminence Alinco Legend. Here is a picture of the rusty crusty thing. The circuit operates just dead quiet even with all the original circuitry and original bogen tubes. It takes my pedals very well. It's really responsive if you crank the channel volume to 10 and adjust your overall volume using the Master Volume. I think it looks and sounds cool as is. I think I will wire in a standby switch and 1/4" speaker out on the 8 Ohm tap and call it good for now. I can always change my mind and mod it later. Platefire
-
Bogen CHB20A's ? I still have about a half dozen of them laying around------ modded/sold some on ebay---some locally----haven't messed with them for a while now.
Here's a poorly done vid demo of a CHB20A (scroll down to see vid)---- done several yrs ago by some friends with a (then new) digital camera that would take short vids---- the camera's mic is terrible (sounds like amp is oscillating, but it's the camera mic)---- pretty loud for the camera mic thru the white Sound City 4x12 w/G12H30's.
This is a modded Bogen circuit, not a gut-and-rebuild to another circuit-----i liked just brightening up the existing Bogen circuit on these CHB20A's----adding adjustable bias, new filter caps, etc, as needed.
Again this is from several yrs ago---- i'd probably do something different today on a CHB20A re-furb....................gldtp99
Link: http://www.myspace.com/gldtp99 (http://www.myspace.com/gldtp99)
-
Thanks gldtp99
I live out in the country/rural area and am on dial up! so it takes forever to download any type of large file--and I just don't have the patents to wait. I did click on the link and it ran a long time and never did open up--but thanks anyway. The only option I got for faster net is Hughes net and I hesitate to pay that on my retirement pay :undecided:
I'm still scratching my head on this bogen cause it sounding pretty dog gone good to my ears and basicly all I've done is wire in a 1/4" input directly to the mic channel using the same old existing shielded cable that ran to the threaded mic input on the back. I ran in through a 1-12 cab tonight with a 100watt Carvin British Series speaker and it sounded a whole lot better than the first 1-10 cab I ran it through. Besides sounding good it also responsive enough to where its allowing me to play pretty good to the point of inspiring me to do a little extra than normal and that's making me pretty happy with it.
I just can't figure why I'm the only one thats liking it as it is??? It seems to have plenty of treble/highs for me. Maybe I'll get over it, I don't know? but right now I'm having to much enjoyment out of it to change it---so I'll just ride it out and see where it goes.
My CHB-20A is missing the folding back metal panel. If you got an extra one of those, I would like to strike up ad deal with you on it. Platefire
-
I was playing humbuckers exclusively back when i was messing with mine----- the CHB20A's would tend to flub out on the bass when played at, or near, max (only way i played them then)---- so i really limited some bass rather than brightened them up---- changed to some smaller value coupling caps, as i remember.
I liked the stock circuit, other than that-----these amps often would come with a Mullard 6EU7 and Mullard or Telefunken ECL86's.
Bogen also made other small PA heads, of the same era, with ECL86 push/pull outputs--- i have a CHB14 (blown PT) and an MU10-- both have tube rectifiers so they don't have the voltage doubler PT of the CHB20A and both have cathode biased ECL86's vs. the fixed bias CHB20A---i keep the MU10 at my shop to show people how a 10 watt head can sound thru a 4x12 cab--- this one has no tone stack, just Bass and Treble cut slide switches and it sounds like a cranked Marshall JCM 800 when dimed---just at a 10 watt vol level.
I'll look for a flip-up back panel but i think i threw them all out when moving my shop---i'd remove them so the amps were easy to carry one handed---i remember throwing an armfull of them in the dumpster a few yrs ago---i still have many CHB100, CHB50, CHB35, CHB20, etc, etc in storage--- if i find one you can have it.
I'm currently re-building a Challenger (by Bogen) CH30 to Fender 5D5 Pro-based specs----except with a cascased preamp and a "balance pot" for the paraphase PI---i gutted the orig circuit and will re-use the orig OT--- orig PT not usable--- new PT from Magnetic Components---- 6L6GC x 2, cathode biased, 5U4GB Rec, three 12ax7's for pre and PI----should be a cool sounding amp.......................gldtp99
-
OK, Thanks! Of course the amp is fine without the backplate. I become kind of fond of the amp since I been jamming on it and have since added rubber feet(that was missing) and also a fender type handle out of extra parts I had. Usually I want to paint these old amps and make them pretty but I like this one in is old rusty crusty state--I think looks cool as is. So I understand about cleaning out old stuff and have done that serveral times myself--you tend to accumulate a lot when you been in this a while. So if you turn up a backplate I would be happy to send you shipping and a few bucks for the plate.
I'm pretty familiar with the CHB line. I've have a CHB-50(thats kind of rare) that I converted to a 5F6A.
The OT was bad and I put one of Dougs bassman type OT in there and man-what an improvement. It sounds and works good but I did it in my early amping days and it's all old style point to point without a board(a lot of terminal strips)and my grounding habits/experiance were non-existant--so the amp has a little ground hum which really annoys me because I like a quiet amp. So I've been planning to go back in there one of these days and see what I can do with it but haven't yet. I also had a CHB10A that I completely gutted, rebuilt with a princeton type preamp/tonestack that was amazingly loud for what it was. I kept it a few years and sold it on e-bay. I tried the CHB20A yesterday with my Les Paul and just cut the bass to about 9:00 and boosted the treble to about 3:00 and jammed on it a while and was happy with it. This was in my living room and the real test would be playing out somewhere with it because at louder volumes really tells the tale. Platefire
-
On this amp, I was wondering about trying relocating the MV after the tone stack prior to the PI but I find the MV pretty useful even in it's present location. Even though in it's present location doesn't have the dramatic effect of normal guiter amp MV operation, it is useful to a certain degree.
I was wondering if you could grab the grid signal just before and after the existing MV location and run it to a DPDT switch, also interupt the grid just before the PI and wire the switch to switch between MV locations? Seems workable but I may be missing something! Platefire
-
Howdy folks, just running through some old posts.
In the preamp stages for the Aux and mic channel there is a 470K Resistor in parallel with a 100(pf?) cap. Is this a high pass filter to increase highs and reduce gain or what is the purpose?
Never caught an answer to this. I was wondering this myself as I am wanting to mess with one of the stock CHB20a's I have lying around. What would be the result of removing this be? BTW I have already canned the aux input and extra volume pot.
Also, I'm wanting to reduce gain. Normally the easy fix for me (being a perpetual novice) is to switch 12ax for 12ay or other low gain 12a? type. Darn Bogen and their 6eu7's! I really don't want to rewire for 12a? since I have a box full of 6eu7. Would adding a cathode resistor/bypass cap on the first preamp stage help?
These little CHB20a's are fun amps. I also have a screamer that Gldtp modded a few years back. But this next one I want to tame down a bit to get more clean headroom. Luckily I've collected a few sets of these tubes over the years since the 6GW8 is getting somewhat expensive.
Thanks
Eric
-
Eric
I know what constitutes a good tube amp is way subjective according to the individual. I'm mostly a clean player that uses the guitar volume like the old blues players and like it right on the edge of breakup with my guitar volume fully cranked. I use my Route 66 pedal for pure distortion. I'm really finding the master volume handy on this stock amp even though it's in a location not normal for guitar amps. For my Tele or Strat I can crank the pre-volume 100%(10), adjust my overall amp volume on the master vol about 1:00 and get my tone and feel. With my Les Paul that has pretty hot 12K Ohm humbuckers with Vol#1 about 8:45 and Vol#2 at 1:00 I get about the same response. So the volumes are actually usable in my case for cleaner sounds and good compressed start of breakup at top volume. Of course this amp will get into big time breakup if you keep cranking the volumes up.
I finding the tone stack so far very functunal for my guitars. I like kind of a fat tone with just enough highs to break through on a leads but thick enough to get a good chunky rhythem sound for just backup rhythem sound. On my tele/strat I'm finding that set on "0" for bass and treble--not boost or no cut. On my LP and humbuckers---cut bass -1.5 and boost treble +2. So you see I still have a lot of play on the dials for more control.
The 6EU6 is sounding oK to me. Even with 58VDC(shows 68 on the schematic) on the first gain stage. Second 6Eu6 gain stage plate is 147. I'm getting nice compressed feel but still clean even when the guitar volume is low. All other voltages on the amp are real close to the schematic listings. Power tube plates are running 395 and are shown 400 on schematic. Screens 374/375 Schematic.
I'm still in the honymoon test out period with this amp but I continue to be impressed with the stock PA circuit. Platefire
-
> a 470K Resistor in parallel with a 100(pf?) cap
Got wire-snips? Clip the cap and listen.
The resistor is part of the mixing system (so sources don't fight each other). There's stray capacitance everywhere. When audio comes through a huge resistor, stray C sucks the highs off. The 100p _appears_ to attempt to correct this. A moment with a calculator says the cap in 100pFd||470K works only on the top octave of useful guitar harmonics (above 3KHz), so may be moot for your use, though important to pass a 50Hz-15KHz Acceptance Test for a contract sale.
-
OH NO! The dreaded stock Bogen is back :BangHead:
I said I would shut up after the last statement--but putting this thing on a nice efficent speaker makes as much difference as daylight and dark. The honeymoon is aging and it's still sounding good. I'll really try to shut up now. Platefire
-
Hello
I'm still happy with the stock Bogen CHB 20, I just would like to relocate the MV to pre-Phase inverter to get higher gain at lower volume---or because tone stack is behind gain stages--would this change not accomplish my purpose?
Schematic is attached. It will just take rewiring the existing R5 1 meg master into that location. I'm just wondering with the MV removed from it's current location, do I need to rewire R13 (1 meg grid to ground) in that location and let R5 take R13's location. I would like to make this change without negatively effecting the existing sound, just better gain control. So if this change won't accomplish this, I would appreciate your input. Thanks, Platefire
-
Well! since none of ya'll came to my rescue, I sucked in my insecurity, prayed and went ahead with the MV relocate. First try after re-wire the MV wasn't working with bad hum. Found the gound on the MV wasn't working and ran a new ground wire---that did it--nice a quiet now.
The master volume sure did what I was hoping for and gave the me more control over gain and overall playing volume. I can now jam at a low volume clean punchy tone with great response. Seems like the original integrity of the amp is maintained with the exceptions below.
On the negative side with the pre fully cranked and MV low the distortion is way flubby for my taste. Even though I wouldn't use this setting hardly any--be nice if it was tighter. Also the overall gain increased to a point where I think I lost some of my overall loudest clean headroom. May need to tweak with a dropping resistor---still open to suggestions on that!
So basicly what I did was not change the existing circuit it any way other that un-wiring the MV in its existing location, connected the grid wire direct where it was removed and wired the tone stacks trebel output grid wire to phase inverter through the MV putting it just before C10 and R13 on the schematic. Platefire
-
cool! glad you got it working.
peace.
--DL
-
I haven't had much time for amping lately. Sure felt good to melt some solder and move a few wires around again. I've been jamming on it off and on all day and pretty happy with the results. Looks like before it's over I will end up putting a MV on everything I got. I've got a Bogen CHB-50 and Dukane PA heads with Bassman circuits and no more room/space for another volume control--I will have to use a ganged pot for those. Platefire
BTW-On the Pignose G-40 they modified the Bassman circuit slightly to better work with a master volume. I'm wondering if I should use that circuit as an example for adding a MV or has anyone had some experiance at adding MV to a 5F6A?
-
Hi all,
I'm trying to play along at home, and I think I'm close to 'schematic-ing' what Platefire is doing with the Master volume.
If anyone is still reading this old thread, could you take a look at this and confirm/correct?
It's my first ExpressSCH attempt- I expect mistakes...
-Ben
-
Annnndd, first post noob mistake!
Here's the much-smaller-file-size image I meant to send with that last post, dangit.
-
Believe it of not I'm back on this thing again. I fired this up the other day and it just seemed to distorted if you put any volume on it past 2. So I rewired the volumes & tone stack more or less like "HeyitsBen" in the previous post schematic was drawn up ---still to distorted.
I decided to start reworking it from the input on out until I get some good results. Tonight I de-soldered the V1 wiring to the old 6EU7 socket and trashed it all. Going to rewire the existing socket for a 12AX7 so I will be able to use all the 12A____variations. So I will be rewiring V1 to be guitar friendly in a stock fender input kind of way. The plate voltage on V1A is 59V with a 470K plate resistor and V1B is 150V with a 220K plate resistor. I was thinking about going with 100K plate resistors and getting the plate voltage up around 250V that would hopefully help the clean headroom. Gosh!!! those old Bogen workers made some tough mechanical connections for their solder joints---it was a dickens of a time getting all that loose on the old socket without destroying it :BangHead:
I was also thinking of going to a more guitar friendly tone stack. I just need to use the two existing 1 meg pots(treble & Bass) with the long split shaft to work with my existing Bogen Knobs and Cover. I was thinking the standard two knob Princeton/Harvard/Deluxe but they all use two 250KA for bass and treble? What effect to the sound would using the existing 1 megs with that type tone stack have as opposed to the fender 250K ones??
Guess I just had the desire to tinker with an amp some--so if I could get a bit more clean headroom and use the MV to get a little grit also would be nice. Platefire
-
aside from a dying tube, perhaps it's a leaking coupling cap or a shorting bypass cap. check the output stage and preamp bypass caps. to test the coupling caps, lift out the grid end of the coupling cap and measure DC: if you see over a few millivolts, replace it.
check B+ taps, if any or all are abnormal (low), check the power supply caps.
--pete
-
Hay pete thanks! I did double check the voltages in the output amp section. One abnormally I found is on the schematic it shows the plate voltage going to the PI V2A 180V but it is reading 275. The power node feeding the PI's reads 280V correctly. Next is the 220K resistors plate resistors. Other PI plate on V3A is reading correctly at 130V like the schematic. The 220K resistor on V2A is only knocking the voltage down 5 volts (280 to 275).
I did measure the ohms in place on both those resistors and they did read close to 220 Ohms. I don't know if it means anything, but one power tube is running hot and the other cool to the touch?
So I went and switched the tubes. Now both PI's Plate voltage at V2A and V3A are now 130V. Both tubes are hot to the touch now---but according to the schematic V2A should be 180V!!
So I got three 6GW8's, one NOS and the two that came in the Bogen. The NOS is in one of the sockets already. So I swapped the used one in there for the other used one. OK I measured voltage again and now V2A is 171v and V3A is 130V plate voltage. Both are hot to the touch. That is better aligned with the schematic. One of the tubes must have been weak.
One more thing that may be an issue. Bias voltage on schematic is -16 but is reading -12. May need to replace those bias caps.
That may have been my problem all along. I can't test the audio until I rewire for a 12AX7. So I may get into that tomorrow. Thanks, Platefire
pdf schematic is attached.
-
Would one of you guys happen to have the Bogen K10 schematic to post here?
-
You mean like this one?
-
One more thing that may be an issue. Bias voltage on schematic is -16 but is reading -12. May need to replace those bias caps.
absolutely: always replace those. just because of age and regardless if they are still working.
with -12V bias check for red-plating with the lights out. the -12V bias may have contributed to premature demise of the cold tube. replace C15 (.22uF), C16& C17 (25uF) electrolytics. note polarity. ;-)
--pete
-
You mean like this one?
that looks familiar! :icon_biggrin:
hacked mine into the attached schematic. kept the paraphase. ditched the tone selector and used "5E3" tone control, and modified 6AU6 bias - originally it was grid-leak bias: it was changed to cathode bias. eventually added a "presence", however can't locate the schematic with what i did.
--pete
-
You guys are the best. :worthy1:
-
here's the original k-10 thread tread i started: http://el34world.com/Forum/index.php?topic=7404.msg167532#msg167532 (http://el34world.com/Forum/index.php?topic=7404.msg167532#msg167532)
let's not clutter up plate's CHB20A thread... :icon_biggrin:
--pete
-
Platefire, here's a preamp that will work very well with that amp. This particular schematic is for an Ampeg Gemini II, but this identical preamp appears in many other Ampeg models from that era. I've just added this preamp to a Hammond organ amp. Sounds very guitarish.
You may notice that it's almost identical (except for component values) to the preamp you just ripped out. :icon_biggrin:
EDIT... added schematic with voltages.
-
Thanks sluckey! I looked through a ton of schematics last night looking for something that would work with what I got and didn't turn up anything. I even looked through Doug's Ampeg schematics but guess I missed that one. So that looks good to me. The only thing I was wondering about after looking at that Ampeg preamp/tone stack is the MV. I may not really need one because I'm mostly interested in clean, but if I was to use the extra 1 meg vol I got, I guess it would go right after the 270k resistor? Platefire
BTW--I haven't ripped the existing tone stack out yet :think1: , only all wiring related to the 6EU7 to rewire for a 12a---types. Also, do you have any plate voltages for that preamp I could kinda aim at?My "D" node voltage is going to be close to 280V.
-
New pic showing voltages and MV. This circuit should work fine with 280V but if you want higher just add an extra resistor filter cap node. Connect it to that 375V node on the schematic. Change the resistor value for whatever supply voltage you want. Ampeg actually used 280V supply on the Gemini II.
-
Thanks Again! I just need to go through my existing inventory and see if I can come up with the different parts.
I think I will go with the existing 280V first to see how it works. I think I discovered one of my main problems last night with a weak or bad power tube and after swapping the tubes around, think I've got a good working pair now. It might have sounded pretty decent if I would have discovered that first but I think the 12AX7 rewire
will make it a lot more maintenance friendly for myself and any future users.
Thanks for the nice schematic! I think other Bogen CHB-20A hackers like me may be interested in that also. Platefire
-
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
-
The 120K between the bass and treble wipers... I've always seen a resistor in that position on james tone stacks. I just always considered it to provide some isolation between the two pots to cut down on interaction? Just a guess.
As for the 270K from bass wiper to ground... I don't see that very often, but it is on the Ampeg Gemini II and several of the B15 amps. But you won't find it on the Gemini I or several other Ampegs either. So, I guess it's not really necessary. Without that resistor, the following grid stage has to rely on the Bass wiper to provide a ground return path. Maybe Ampeg put the 270K there to provide a dedicated ground return in the event that the Bass wiper fails? Again, just a guess.
-
just a guess
I'll accept your guess, thanks sluckey
-
sluckey
I'm looking at parts to accomplish the preamp on the schematic you provided. I'm confused about V1B cathode bypass .1 cap. I can't find an electrolytic cap with .1 value. Is that suppose to be a regular coupling type cap?
Platefire
-
I can't find an electrolytic cap with .1 value.
Me neither! :wink:
-
> What's their job?
The B and T on a James always interact. This helps reduce that.
The value makes very little difference. For impedance range as in Sluckey's diagram, 100K to 330K just audible. Higher values tend to shift turnover point to lower frequency.
270K from bass wiper to ground gives less bass at "5" (or "0" hi-fi knob scale). This may be to shim the "flat" setting when you can't get 10%-taper pots (many pots today are 20% taper). Or maybe to voice the amp. I'd think you could omit it and just set the bass knob to-taste.
-
No .1 electrolytic to be had :BangHead: also the bias has a .22/400 cap coming off the PT Red secondary can't find no such animal either :w2: are these caps of the past, no longer available? Platefire
-
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.
-
> 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.
-
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! :laugh: Platefire
-
Rather large aren't they?
-
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
-
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.
-
sorry plate, should have stated this a bit more clearlyr:
C15 - .22uF film cap.
C16 & C17 - 2 x 25uF electrolytic caps.
:icon_biggrin:
use a 400V for the .22uF and 25V parts for the electrolytic caps.
-
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
-
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. :icon_biggrin:
--pete
-
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? :w2:
-
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? :w2:
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
-
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 :dontknow: 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 :dontknow: Platefire
it's fine. slight bump at 400Hz center when controls are set to flat.
--pete
-
Well new preamp and TS coming up, guess I'm good to go to "TINKER CITY" :happy1:
-
I had a rude awakening last night! :think1: 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
-
he, he, he. I've fixed stuff in my dreams too! Sometimes I even remember the fix the next day! :laugh:
-
nobody installs a 47K cathode resistor
I downloaded Sluckey's preamp, looked at it twice, and was gonna post "why 47K" thanks 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. :icon_biggrin:
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
-
Now let me go fire it up again and jam a little>>>>>:>O Platefire
:guitar1
-
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
-
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. :icon_biggrin:
The more signal you let through the more it pushes the tubes.
-
glad you kicked some life back into that old thing.
pics? sound bits? :icon_biggrin:
--pete
-
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.
-
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
-
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! :icon_biggrin:
-
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! :icon_biggrin:
+1
+ nice work.
--pete
-
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
-
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?
:dontknow: 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!
-
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?
-
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.
-
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?
-
try changing R24 from 12K to 22K.
--pete
-
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
-
glad she's humming along now... it would be cool to hear it as well... no pressure! :wink:
--pete
-
Is there any real simple way to record a clip--like on my smartphone :think1: 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
-
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, 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
-
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
-
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...
-
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. :BangHead:
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 :laugh: Platefire
-
Do you want to make the bias adjustable? Or convert to cathode bias?
-
> 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.
-
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
-
>> 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
-
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
-
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
-
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
-
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
-
AES sells 6GW8s for $40.
-
> 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.
-
DummyLoad requested a sound clip
This my first attempts at you tube. I had the amp turned way down low, wife was talking on the phone--so I did something to keep from doing nothing :dontknow:
! No longer available (http://www.youtube.com/watch?v=lMbdjF9iGlw#)
-
thank you! very nice and laid-back tone. it's a keeper as-is me thinks. i'll put a finger on my NOS suppliers if you wish; please send me a PM if so.
--pete
-
Thanks pete
I've actually just purchased some on e-bay today. There were two NOS/NIB GE & Admiral 6GW8's for $49.99.
I offered $30.00. He came back with a counter offer of $45. He had $3.95 shipping on them, so I came back with a counter offer of $45 to include shipping, so I actually got them for $41.05. There is one hitch though, MIJ!! But the NOS Lendel that I put in there to replace the bad Bogen tube was also MIJ--so they will be in like company. I may of did bad! but I did something :dontknow: If those tubes made for Bogen had lasted all the years with that intense plate voltage, maybe these will to? Here is a picture of them. Platefire
-
Thanks pete
I've actually just purchased some on e-bay today. There were two NOS/NIB GE & Admiral 6GW8's for $49.99.
I offered $30.00. He came back with a counter offer of $45. He had $3.95 shipping on them, so I came back with a counter offer of $45 to include shipping, so I actually got them for $41.05. There is one hitch though, MIJ!! But the NOS Lendel that I put in there to replace the bad Bogen tube was also MIJ--so they will be in like company. I may of did bad! but I did something :dontknow: If those tubes made for Bogen had lasted all the years with that intense plate voltage, maybe these will to? Here is a picture of them. Platefire
you've done well. the MIJ tubes are likely either hitachi or matsushita/national and either are as good as euro or USA made stuff. i'll query my contact anyhow, i'm curious to see what he has, if anything. he mentioned that they are used in some HAM radios and i remember him telling me that they were a prize tube for HAMs as well.
--pete
-
Thanks, it makes me feel better that I might of made a good selection. If you look at the construction of the tubes very carefully in the picture, they look identical even though they got different brand names. This is the first time I've used the "Make an Offer" feature, kind of like it. Platefire
-
PRR
I hear you on the plate voltage. A 100V over is more than I would purposely design/build an amp. Makes me wonder what Bogen was thinking in their design? But also, how did these vintage tubes last all these years? most of them still have their original tubes in them. Platefire
-
I found a guy who modded his CHB-20A by lowering the voltage 110V using diodes. Attached is a picture of what he did. Is this a better approach than using dropping resistors> Never seen this used for this purpose before. He said he drilled some extra ventilation holes by the terminal strips, so must be considerable heat involved.
I'm not sure yet if I want to do this on this amp because I'm wondering how much that would effect the clean headroom I'm presently getting. But on the other hand, if I end up using this amp a lot and tubes start going down the drain I may need to do something. Just a consideration that I'd like to understand better. Platefire
-
that amp ran reliably for decades with 400V with those tubes. i'd leave it be. just my 2cents... :icon_biggrin:
--pete
-
Amen! I think I will take it to Church tonight and hook it up to my Boogie 4-10 cab and if all seems good, use it in Church services tomorrow with that cab. Platefire
-
I did take the Bogen to Church and practiced with it hooked up to my Boogie amp(see Pix) with a 4-10 cab. It has Jensen Special Design Alnico speakers and are wired for combined 8 Ohm load.
The Bogen sounded great through the Boogie cab and handled the 4 10's fine. I left the MV wide open and just used the pre-volume as my only volume with my LP. The amp has wonderful round tone just set flat on the tone stack. I tried boosting the treble a bit but it was too much, much better flat. When I got through and was shutting down I noticed the volume was on only "3". It took my pedals fine---had to do a little pedal tweaking on them but that's normal for any change up. I practiced with the wife and my JamMan but looking foward to using it with the full band tomorrow. Very Very Pleased with how it sounded and operated tonight. Platefire
-
I think you got a reasonable deal on those tubes. :icon_biggrin:
They are not common tubes and I couldn't tell for sure from the pic, but they appear to have the Philips X on the top.
HBP pointed out that MIJ tubes with that X were made on Philips manufacturing gear.
To me, that indicates that these could also be considered premium tubes.
Your amp sounds great to me! :icon_biggrin:
-
> I left the MV wide open
I suspect the Bogen was intended to run with Master around "5". It's not a strangler but a gain-balance.
Then you would use more of your Preamp knob.
For guitar, you might try that and then disagree.
-
Paul, thanks for another conformation on the tubes. I looked at the pix and didn't see the "x" you were referring to. I found myself looking in the back of the amp as I played it this weekend even feeling them with my finger tips. No red plating or outlandish heat. The sound was great, so I think we nailed the bias, of course with a little help from my friends. So hopefully these tubes will hold me for a while.
PRR----I'm so impressed with the pure clean tone on this amp is the reason I set the volumes that way. I will get around to dialing in more feel by rolling back the MV but that clean & my LP is sounding so good I'm just smelling the roses a bit. I do have some good dirt boxes to stomp some in as needed, but yeah I want to get around to dial in a response where I can use my guitar vol to go between clean and slightly edgy breakup. I've already sampled it a bit and it's good. Platefire
-
I've got s schematic cooking for this one. I think it's a pretty good recipe worth documenting. Based on past track record, there will probably be unforeseen changes down the pike. I'll post it when I'm done and request you look over my shoulders, sometimes it's hard to see your own mistakes :icon_biggrin: Platefire
-
the "X" are seams in the molding of the glass. you see seams that appear as crossed lines where the glass bulb was melted together to seal the top of the bulb.
--pete
-
pete--I'm assuming you are talking about that little tit at the top of the tube? I can't see it. Maybe when they come in I can see what your talking about :dontknow: Platefire
-
run your fingernail around the top of the tube next to the tip. if it's an amperex, you'll feel four lines on the glass. look at it close you can see the lines they form a cross across the top of the tube.
--pete
-
pete--I'm assuming you are talking about that little tit at the top of the tube? I can't see it. Maybe when they come in I can see what your talking about :dontknow: Platefire
Take a look a the tops of these tubes.
Old Mullards, Bugleboys, Amperex also have these lines.
I forgot that Uki had a great picture of that X. Thanks Uki!
-
Lines on top of the tube like making an X? See if this is what you are talking about ?
-
Thanks uki
I can definitely see the X on the pixs you linked. I can't tell on the pix of the 6GW8's I ordered but now I know what to look for. I'm learning a lot about NOS tubes lately. I also been watching the discussion going on about
12AX7's, which ones is best. I have quite a few used vintage USA tubes but very few NOS anymore. NOS's will be going up and up as they have been and any good deal you can get on a NOS/NIB I think is a good investment. With that said I just threw away a whole hoard of NOS/NIB Television tubes that I couldn't sell or even give away. So you would need to chose and pick which NOS/NIB to invest in and that will take a little NOS education which I'm getting little by little. Platefire
-
My NOS 6GW8's came in today. No X seam on the top. Platefire
-
Well I finally got the schematic finished. I've already located a couple of mistakes but I'm sure there is more.
Please check it out and let me know of any errors you find. When it appears it's done, it can go in schematic library.
I've attached the SCH file, also the original Bogen schematic and also sluckey's schematic of the Ampeg Gemini
II preamp. Bare in mind some of the values on the caps in the preamp, I didn't have the exact value and used what I had. The ones I actually used is what recorded as an as built schematic. Thanks, Platefire
Note: On V1A I've got the cathode marked pin 1 and plate pin 3 incorrectly. I have corrected that.
-
UH OH! Left the standby switch off. So here is the new version. Platefire
-
Crap! I thought you had come to your senses. :laugh:
-
Dream on :sleepy2:
-
I've been reading this entire thread from a few years ago, I just got my hands on 2 of these Bogen CHB-20As, one of them is in a 19 inch rack mount case. I'll start a new post on this once I get into the amp, hopefully soon.
-
Well My! It's been ten years since that mod. That pre-amp mod sure made a world of difference for the better in that amp. I highly recommend that mod for that amp. Hope all you old ampers are doing good. Platefire
Edit: Sorry, Just saw mod was done in 2015---only 6 years old :dontknow:
-
Once I get into my CHB-20, I'll have to decide if I want the preamp configured like the Ampeg Jet 2, or if I want to put the tone controls between the first and second stage triodes. I'm not sure which would be better...