Roberts 1630 6BM8 based, stereo practice /recording amp

[Toxophilite]

I want to mix the guitar sound from the left channel of the amplifier in this build into the right channel of the same amplifier.


I believe with passive mixers I end up having the mix be both ways.


Ideally I do not want that. I just want to mix some of the left channel into the right channel power amp.

[Toxophilite]

Good to have a healthy obsession that keeps one off the streets.


As it appealed to my streamlined approach to this amp and I thought something different with a strong 'stereo effect' would be cool I installed the Ampeg echo twin style verb. It doesn't need a transformer, only one triode, small component count, and 'stereo'


The left channel OT powering the tank and sending it back into the right channel at the 6BM8 triode. Works great as tested before. VERY FULL sounding, which is cool but sometimes overpowering on the bottom end at higher levels of verb.


Switching the cap on the plate of the recovery stage 12AX7 triode from .0047 to .002 or even .001 gives one more of a brighter fender sounding verb and still plenty of it.


Would it be too nuts to have a switch so one can switch between the two sounds of reverb?
Sort of an Ampeg/Fender reverb switch.


I was really tempted to build both circuits in to the amp as there is room and have two different verbs for kicks..but I worry it might be a little noisy, especially feeding one into the other. I was goofing around with it briefly and it sounded pretty cool.

[2deaf]

Would it be too nuts to have a switch so one can switch between the two sounds of reverb?

It's getting harder and harder for me to draw a line between too nuts and acceptable nuts, but switching the capacitance of the coupling capacitor has been done innumerable times.  More than one way to do it, also.  You could switch the capacitance of the cathode bypass capacitor, instead, to achieve a similar goal.  You might want to lower the capacitance of the bypass capacitor in any event.  A lower value capacitor will cut the reverb faster when the footswitch is used.

Loading the reverb tank output with a 22K resistor rolls off the upper highs.  You might try a 100K resistor or larger to see if you notice any difference.

 

[Toxophilite]

Acceptable nuts, this is where a squirrel comes in handy as they can tell if a nut is unacceptable without having to crack it open.


Thanks a lot for all the ideas and options. I will enjoy trying those out


Part of what I'm hearing is the short 3 spring generic tank I'm using due to use due to the size of my 'cabinet'.
 I might build a purpose built cabinet eventually.
The Ampeg circuit has more top end sizzle and splash with a long fender style tank.

[Toxophilite]

Reverb sorted! Thanks!


I have a question regarding the preamp outs!
They're hooked up to the plates of the 6BM8 triode with a .02 cap. They seem pretty hot and the right channel distorts


Is this a less than ideal preamp out for going into a board ? is there a way to make it happier?

[PRR]

> the preamp outs!

Signal from drivers to power tubes is over 10V.

You want less than 1V!

No doubt it distorts hotly. You are overloading what you are going into.

[Toxophilite]

Thanks
any idears?


I was contemplating switching the 'preamp outs' to 70s fender style line outs coming off the speaker jacks.
2.2k in line(series),  with a 270 ohm to ground

Unless somewhat has a better solution to use with the preamp outs?

[shooter]

any idears

since you are designing more like audiophile than guitarile you need to limit the input signal to the desired output, can be done anywhere before the input jack, otherwise you'll have to do voltage dividers and scope the path.  OR dump gain stages for CF's might get you there

[Toxophilite]

Hmm
I am building a guitar amp. But one intended mostly for clean playing, like the way most of the classic amps were designed.


My 1980 silverfaced super (last of the silverfaces) has an effective line out off the speaker jack that I've incorporated into every amp build I've done.
I find it very handy late at night when I have a song idea I need to work on but can't actually mike my amp. I can get quite a nice sound going straight in that way. Going straight in with stereo reverb and echo etc. is even better


I was curious if anyone had an idea about an effective way to achieve the same end using the preamp outs but tweaked?

[sluckey]

Just put a 10:1 voltage divider on the preamp out jacks. I'd start with a 100K and 10K. Maybe even make the 10K a pot.

[PRR]

> any idears?

What does each amplifier stage do?

Make signal bigger!

If signal out of stage 13 is too big for your needs, move the tap back to stage 12.

--- also --- tapping signal out of the INside of a NFB amplifier is "bad" because the internal signal is trying to correct the distortion and response of the amplifier. Not a ton of NFB here on a 12AX7+"6AQ5" with 2700:47 NFB, but still not the best idea.

BTW, that signal at the 47 is more guitar-level and VERY low impedance. Taking headphones here would screw-up the amp, and the DC here may upset external loads, but 10uFd+470r from that point to a jack may make a fine output.

[Toxophilite]

Interesting and thanks.
Perhaps I should've been clearer.
Anything that will allow the effects to come through too. In this instance I believe it would have to be at or after the triode stage of the 6BMB. Tremelo is inserted at the cathode of the left 6BM8 triode. Reverb comes back on the grid of the right 6BM8 triode


Also I would like to be taking a similar output from both channels


I know the late 70s fenders are often scorned but the line out on the speaker output worked pretty well considering.

[sluckey]

I know the late 70s fenders are often scorned but the line out on the speaker output worked pretty well considering.

Well, just do it. Or you could try this...

[Toxophilite]

Cool I will try a few of these options


Thanks

[Toxophilite]

Tried 470 to line out and 47 to ground on the pentode grid (517k total to ground). Sounds good but very quiet, Needs almost all my preamp gain to get a strong signal to the computer via the board. For fun I tried swapping the resistors to ground, 56k, 68k, 100k. Signal got progressively a bit louder, probably completely workable as it is digital and it doesn't need to be a really loud regardless.


Also tried inserting a 100k in the original preamp out arrangement with 10k to ground.


Louder signal, works well. At least as well as the other fender arrangement I mentioned.


Is this 110k running in parallel to the 470k to ground on the grid of the 6BM8 pentode?

( I had to replace the original 500k with a 470k resistor as the leg broke at some point. Will fix the schematic)

[Toxophilite]

To confirm the implementation of this:

Hooking up to the junction of the 47 ohm resistor and the 1.5k resistor on the left channel 6BM8 triode?

The 10uf cap + 470 ohm resistor arrangement, in series?


See schematic screenshot in red 'PRR-amp output'

The only 10 uf caps I have handy are polarized electrolytics

( I mistakenly tried it with a .1 cap in series with a 470 resistor tied to the junction of the 47 and the 1.5k resistor and got a strong and very crisp signal)

> any idears?

What does each amplifier stage do?

Make signal bigger!

If signal out of stage 13 is too big for your needs, move the tap back to stage 12.

--- also --- tapping signal out of the INside of a NFB amplifier is "bad" because the internal signal is trying to correct the distortion and response of the amplifier. Not a ton of NFB here on a 12AX7+"6AQ5" with 2700:47 NFB, but still not the best idea.

BTW, that signal at the 47 is more guitar-level and VERY low impedance. Taking headphones here would screw-up the amp, and the DC here may upset external loads, but 10uFd+470r from that point to a jack may make a fine output.

[Toxophilite]

This provides satisfying mixing of the left channel into the right channel without visa versa. It does seem to emasculate the right channel some requiring it to be turned up a fair bit more to achieve the same result from the right channel input.

[Toxophilite]

This seems to work better for mixing so far, Anybody see any potential problems with the left to right channel mixing circuit? IS 220K optional for mixing for some reason?


I noticed there were a lot of mistakes in my schematic. Kind of embarrassing.   One was that somewhat due to cutting and pasting.
 I forgot to show that the right channel 6BM8 triode gets it's B+ from node B via a 250k resistor. Which is the original way Roberts had the power configured. For some reason I changed the left channel 6BM8  triode to the C node with a 100k resistor. It seemed to make sense as I was thinking of that channel as a vibrochamp.



I took 500k pot out of the mixing circuit as it seems to be best maxed and seems to sound the same either way.

[Toxophilite]

I've been playing this amp around the house for a while and I like it. However it never seems to have the same sparkle and prettiness my deluxe build has. I thought it might the little 8' JBLS but I tried one with my deluxe and it sounded great.


The amp seems, in general, to lack top end.
is this something inherrant about the vibrochamp circuit or just my strange configuration of it?
Is it the reverb ciruit or lack thereof. I have the Ampeg echo twin 'stereo' transformer driven reverb.


Any suggestion?

[shooter]

The amp seems, in general, to lack top end.

might have tried these, but If not;

disconnect the nfb and see if it moves in the right direction
boost the PA bias (If possible) to close to 12W/tube , right direction?
boost the 25uf bypass to 47uf, right direction?

at the end of day you can squeeze just so much from the little ones 

[Toxophilite]

Thanks. I had some success with a  4.7K resistor in the feedback circuit in lieu of the stock 2.7k required by a  vibrochamp.


Now to further reveal my vast ignorance.


I'm curious if negative feedback looks are designed around the output tap of the OT?


For example if one were using an OT with an 8ohm output tap with a vibrochamp circuit would one use the same negative feedback loop circuit? (same value components etc.)

[SILVERGUN]

This is helpful:
https://www.aikenamps.com/index.php/designing-for-global-negative-feedback

Scroll down to:
Using a different impedance tap on the output transformer

[Toxophilite]

Thanks. I found that link while I was trying to read up on the idea and couldn't open it. Now I can thanks and it has provided some insight though for some reason I can't quite wrap my head around it yet.


I will keep trying and hopefully it will penetrate my thick skull. I do want to understand.


Out of curiousity would someone mind explaining how (and if) a negative feedback circuit in a vibro champ build would be different if one was using an 8 ohm instead of a 4 ohm OT

[shooter]

doesn't matter that it's a vibroXXXXXXXX or a voxxxxxx
the section SG pointed to shows the square n root stuff, so you take the "original" values fender used, plug it into the equation using the new tap.
me, I scourge through schematics til I find "it", then tweak it to taste.

 

[SILVERGUN]

Out of curiousity would someone mind explaining how (and if) a negative feedback circuit in a vibro champ build would be different if one was using an 8 ohm instead of a 4 ohm OT

With all else being equal the 8ohm OT will provide more feedback voltage than the 4ohm'er.

As tweakers, we have been known to replace a feedback resistor with a pot to experiment.

[blackmountainamps]

Hi Toxophilite,
I am new in this forum and got to this thread while looking for opinions on vibro champ vs Gibson GA-5rt circuit. Coincidentally the reason I am curious about those two circuits is because I was lucky enough to get my hands into a handmade (heathkit style) stereo amp that has 2 6BM8 tubes. The amp already sounds great as it is but I want to spicy it up a bit and I had very similar ideas to the ones you had. I converted an old radio that used a 35L6GT power tube into a Fender Bronco/Vibro Champ a while ago and I loved its sound. I later sold that amp. So now I wanted to use parts from this stereo amp I got and build another one. I was thinking about building one channel with the vibro champ circuit and the other with the gibson ga-5rt but I never heard one of those gibson amps before. So... not sure if that's what I want. :) I was wondering if you have any sound clips to share about your "frankenstein" amp experience :) Thanks, Gui.