Bogen CHB35A rework

[ORocket]

Hello, I have a Bogen CHB35A http://tdsl.duncanamps.com/pdf/bogen_chb35a.pdf that I'd like to convert over to an EL84 based guitar amp. I'm not clear on the effect the impedance difference between the original 7868 tubes and the proposed EL84 tubes will have on the sound and output transformer. I believe the 7868s zOut is 6600 and the EL84s in pushpull will be 8000. Is this do-able? I realize the power supply will need to be reworked as well to accommodate the EL84s. Also, what to do with the 6C4 driver tube, punch chassis to fit another 12ax7 or is there a more suitable replacement for the original 6C4 socket?

[sadgod]

I have the same amp. I was thinking about doing something similar. very similar. Then I realized how nice of shape it was in, cosmetically anyways. There's plenty of di-electric debris though and replacing damn near every component is on the agenda. So I'm modding the tone circuit and possibly cascading preamps but leaving the tube arangement since I have replacements. Then I was going to build an amp based on that design but more of the tube arangement you described. Depending on the sound of the origional, I might just leave it with its clean sound attributes and run a pedal or two through it. God Knows how much harder it will be in a few years to get a hold of some replacement tubes though. I probably will need to do a tube mod eventually. Keep me updated.

[Rob Mercure]

Unless your output tubes are bad and cost is a consideration - why would you want to replace the 7868s with EL84s? The 7868 is electrically identical to the 7591 and these are some of the best sounding outputs IMHO. But I'll be upfront and state that there are very few EL84 amps that I've liked. In addition to your OT impedance issues you're also probably going to have to lower your overall "B" supply voltages - perhaps by adding tube rectifier such as a 6X4.

I'd just keep the 7868s - they're being made in Russia now - and concentrate on the preamp stages which are probably gridleak bias. The 6C4 split load PI might prove to be acceptable - I'd cobble up preamp stages to my liking first - but you could, perhaps, just a 6J6 to create a LTP PI - it's a 7 pin miniature dual triode with a common cathode which would fit the 6C4 socket (if you can cleanly remove the old parts tied to the 6C4 socket). Oh, the 7868 sockets have a "footprint" much more like an octal tube than a 9 pin miniature so if you do decide to use the 6BQ5 (boo, hiss <grin>) then you'll have to punch a piece of sheet metal for the 9 pin socket and then rivet it over the 7868 hole. Again, I'd stick with the 7868 but if you insist on changing the outputs an octal socket would fit well and the 6V6 is a much better sounding tube (and if you later change your mind you can rewire the sockets to 7591s and retain your original impedance match).

My $0.02

Rob

[ORocket]

Thanks Rob, sounds like a good start. I do have the original tubes. I'll post again once I have made some progress with the preamp.

[ORocket]

Marshall JCM800 2204 Preamp

OK, I kept the original 6C4 driver and 7868 power tube circuits from the Bogen and wired up a "blueprinted" Marshall 2204 preamp using 2-12AX7s http://www.schematicheaven.com/marsh...l_50w_2204.pdf While it does sound Marshall-ish, older ACDC like, the amp doesn't have much sustain or touch response. The crunch is a bit dirty/harsh. Are these symptoms of the split load driver from the Bogen?

[stokes]

I would re-do that PI with a 12AX7 like the Marshall has.Also it looks like the Bogen has a sort of corrective filter across one side of the OT,I've never seen it wired like that,across just one side of the OT,might want to lose that as well.

[bob p]

you might be able to dispense with the conjunctive filter without any problems. otoh, there might be some good reasons to keep it.

the conjunctive filter was commonly used in tube era hifi circuits in order to provide high frequency and transient stability. it acts as a load limiting device. speaker systems exhibit rising impedance at higher frequencies. some negative feedback amps would oscillate at these frequencies as a result of phase angle changes in the OT and amplifier circuit. (think about how frequency response curves in tube amps have humps in the ultrasonic and subsonic regions.)

to counteract the phase induced oscillations, an RC circuit would be installed across the OT secondary . the cap value is chosen to prevent the loading of the amp throughout the audible portion of the spectrum while still providing suitable loading at ultrasonic frequencies. this would assure stability of the amp under all dynamic operating conditions.

this is one of the textbook approaches to phase compensation in an amplifier. the amount of ultrasonic phase shift, and the parasitic oscillations that ensue, are ultimately determined by the amount of feedback in your NFB loop and by the construction of your OT. well designed transformers exhibit less phase variance at higher frequencies and are less prone to these problems than poorly designed transformers, so they may be more likely to be of help in a "lower quality than HiFi" OT like you'd find in many old PA and guitar amps. bearing in mind that the manufacturer of your OT included a conjunctive filter in the design of a PA amp that used that OT, maybe they knew something about the phase characteristics of that OT at high frequencies, and maybe there's a good reason to keep it.

the good news is that the conjunctive filter is designed to pass frequencies in the audible range while snubbing frequencies in the ultrasonic range. in all likelihood, the conjunctive filter in that amp is tuned to work at frequencies that are beyond the limits of your hearing, and is designed to be transparent at frequencies that you can hear. i think its unlikely that you'd hear an improvement after removing the conjunctive filter, given the limited bandwidth of the guitar. in all likelihood, one of two things would happen after removing the filter:

a) nothing
b) HF oscillation

HF parasitic oscillation might manifest itself as low power, or dullness in the sound. if that happens, just put the CF back on.

one final comment as food for thought: Dr Z puts conjunctive filters in his guitar amps for precisely the reasons i've mentioned. even in guitar amps, they're not all bad.

have fun!

[stokes]

Bob,I've done some experimenting with conjuctive,or as thay are called in the RCA tube manual,corrective filters,(yeah,I know,what am I doing in a book)and they are beneficial,no doubt.I have them in two amps I built that have Thoradson and Stancor OT's,I have found they affect more than just the "unheard" frequencies,they give the guitar amp a much better "balanced" feel.Like all the strings resound more evenly.They cut the ac volts seen at the output slightly,which you would think and actually do affect the output watts,yet it still sounds louder than without it. It just struck me as odd that it is used on only one half of the OT primary in the Bogen.What do you make of that?Cant see anything different between the 2 sides of the PP that would warrant the need for a filter on only one side.What am I missing there?

[ORocket]

Conjuctive Filter

It appears the conjuctive filter in the Bogen schematic http://tdsl.duncanamps.com/pdf/bogen_chb35a.pdf does not exist in my amp. The blue wire from the OT ties directly to pin 9 of V4 as does the brown wire on V5. There is no resistor or cap on either wire. I'm wondering if the omitted conjuction filter is indeed causing HF oscillation and contributing to the dull sustain/sound. I haven't had the chance to add the filter, question is do I add the .0012/3.3k filter as shown in the schematic (1/2 side of OT) OR should I go with a .022/10k filter across both legs of the OT? Also, there aren't any 1k resistors as pictured on pins 2 of the 7868s. The 220k/.047 junction from the 6c4 driver actually ties directly to pin 2 on both 7868 tubes. Is this an issue too?

[Rob Mercure]

OR,

The 1 K resistors are the "grid stop" and will both contribute to helping with oscillation and prevent grid blocking on the outputs - which will contribute to nasty sounding distortion. As to the question you posed several posts ago the PI probably isn't your sustain problem as many amps using the split load PI have great sustain - I'd look at the tweaks in your preamp section first. But if you've got a 'scope this would help narrow the problem.

Rob

[bob p]

the conjunctive filters that i am familiar with are normally placed across the secondary side of the OT, between the highest Z tap and ground. offhand i don't remember any designs that use a series RC filter going from the primary CT to only one of the plates on a pentode mode application (like the bogen schematic).

[stokes]

If you look in the RCA Receiving Tube manual there is a section on corrective filters explaining how to determine the cap/resistor values using it across the primary of the output tranny,which I believe is the one DrZ uses,I also saw an article somewhere describing the circuit Dr Z uses,and referencing the RCA manual.