bogen chb-35

[vanman]

Hi Guys,
this is my first post, so please be gentle with me. I'm a guitar player and I decided I wanted to start building and fixing tubes amps and it's become quite an obsession. So much fun and the possibilities seem almost limitless, (sort of like playing the guitar). Most of my free time is now spent learning, by reading, and of course by jumping in. Good thing I'm single now. This forum has been a great resource and for months I've been perusing. Finally decided to actually post. My latest project is a Bogen CHB-35. I have been using this as a learning tool. Instead of just gutting it and putting in a stock circuit, which I've already done before, I've been playing with various circuits and components to see what happens and how it changes the tone and headroom. I know there's been previous threads about these. Wasn't sure if I should add to those or start a new one. oh well......
Now to the meat:
the heater (couldn't remember how to spell filament) voltage is 3.2 volts AC. I was expecting 6.3. Is this a problem?
The plate voltage on the 7868 tubes is 490 volts. In fact all the plate voltages are high. The schematic calls for 450. I understand this is because the line voltage is higher now. Should I try to bring this down (or leave it as is) and how? Add a large wattage resistor on the B+?
To make this more guitar friendly I was going to do the following:
change the split load PI to a long tail by changing the 6C4 to a 12Ax7.
Use a marshall front end. Since I understand that the 7868 is sort of in between a EL84 and a 6V6, I was going to use the front end of a Marshall 18 watt.
since I have an unused triode, I was thinking about cascading 2 gain stages. From reading, it appears I should put an attenuator network in between the two. Again, doing this to learn, not necessarily because it will stay this way.
this amp seems to have potential. already seems to lean towards the marshall type of breakup, maybe because of the 7868 tubes.
Also, what type of soldering iron and temperature range is appropriate for this application. I've been using a 25 watt Weller.
Well, just wanted to thank you guys for this forum and " hope I passed the audition".
PS. I know "vanman" probably makes no sense. It's because of my 71 VW van.

[Eli Duttman]

Other than basing, the 7868 is the same as the 7591. It's as easy to drive as an EL84, while delivering the same power as a 6L6 (not 6L6GC). O/P trafos that work with 6L6s (typically 6.6 KOhms end to end) work with the 7591.

Remember, a LTP has gain, while a "concertina" actually has a slight insertion loss.

Current production 7868s cost more than current production 7591s. When the tubes you have now wear out, installing Octal sockets and switching to 7591s could be a good move. It's a question of sufficient room, as EH 7591s have bigger bottles than EH 7868s.

[vanman]

OK. You don't have to be be too gentle. If I screw up, I want to know so I won't keep doing it.
thanks for the info on the 7868. I know very little about these tubes.
I thought the LTPI actually had more gain than the split load. Can you explain?

[vanman]

I tried a used set of Sylvania 7868 output tubes and this is what happened. amp started making crackling noises with one tube flashing blue with the crackles. amp then lost volume and tone. put the original tubes back in and amp is doing the same thing. works good for about a minute and then crackles and loses volume and tone. checked my voltages to the output tubes and they are the same as before. higher then spec, 490 instead of 450 but has always been like that and has worked fine. checked resistors in the power supply to the output tubes and all are good. checked impedance on the primary of the output trans, (blue and brown wires) and it was only 350 ohms. shouldn't it be around 5K? did the output trans blow?

[TheGimp]

That is the DC resistance of the transformer. The impedance has to be measured with an ac signal so the transformer is probably ok.

I'm currently converting a CHB-10A to guitar use. It uses the same 7868. THe CHB-10A is a SE amp, so it only has one 7868.

I'm adding a Fender style tone stack in place of the original tone control, and rewiring it to use the first stage as a cathode biased input instead of grid leak bias. I'm locating the tone stack between the it and the driver tube.

Re-check the filament voltage. It should be 6.3 +/- o.3 VAC.


I'll check my B+ and screen voltages tonight and et you know what I'm running.

[Tom Bavis]

You can use a 6J6 dual triode as the long-tail phase inverter - it takes a 7-pin socket like the 6C4.

[vanman]

follow-up post. after trying a used set of sylvania 7868 output tubes the amp started crackling and then lost volume and tone. One tube flashed blue with the crackles. Put original tubes back in but now the amp still does the same thing. I wonder if the 490 plate volts was too much for the sylvania's and what damage it did to the amp. I was wondering if because the amp works fine for a couple of minutes before crackling and losing volume and tone, if that could be the filter caps. they are the original caps. am waiting for parts to change them.
thanks for the info on the 6J6 for the PI. one good thing is the NOS tubes are cheap. found a pair of tungsram for $16.
funny thing about the filament voltage being 3.2AC. that is the voltage listed on the voltage chart of the schematic. I also thought it should be 6.3. makes no sense to me.
I also changed the first stage to cathode biased. much better tone, and am working on the tone stack. waiting for parts.
thanks for your help.

[Stormrider]

Are you sure you're measuring across the entire secondary? Sounds like you are measuring between the heater winding's center tap and one side. I only ask because measuring from the CT to one side would give you exactly half the voltage, 3.15vac.

I have a CHB-35 too. Mine was pretty well toast when I got it, and unusable as is. I still have the chassis, iron and pair of working but mismatched 7868's though.

[vanman]

thanks AJ,
that is exactly what I was measuring. seems like on other amps I measured it the same way and came up with 6.3 volts. Those were DC. Don't know if that makes a difference.
funny, my CHB-35 was working when I got it and is now unusable (for now).

[vanman]

how do you measure the impedance of an O/T.
thanks.