6BM8 Stereo SE Amp

[TIMBO]

Hi guys, First off thanks to Toxo for doing the leg work on this
Roberts 1630 6BM8 based, stereo practice /recording amp (el34world.com)
This is what I started with

and this is where it is now with a few tweaks



Works well and loud enough for a couple of watts.
Two issues.
I have a BUZZ in the main amplifier.
It's not a hum and it's only in the one channel
Rolled a few tubes and no change
Without anything plugged into the input the buzz can be heard faintly with the volume turned up full.
Grounding the input side of the 47k grid resistor the buzz remains but grounding the socket pin side of the 47k kills the buzz completely.
The 47k resistor is connected as close as possible to the pin as I can.
The socket is close to the PT, tried a tube shield, didn't help.
But it does not seem to affect the other channel
With guitar plugged in the buzz increases with volume.


Blocking distortion
I think Toxo was having the same problem.
Did several things...
Changed the volume pot to 470k
Dropped the grid leak resistor on the power valve to 100k
Increased the grid resistor on the power valve to 470k but returned it to 47k as it didn't do much
Put a 100k bleed resistor after the volume pot.
These changes helped without loosing to much volume.
The one thing I wanted to try was the speaker driven reverb.
Works very well with plenty of boing.
It does not affect the sound of the main amp.
It is kinda different to the usual reverb as follows the chords/notes in an almost delayed fashion.
Is there a reason this didn't take off.....

[uki]

Buzzing like ?!  Are those transformers old? Sometimes transformers vibrate due to the frequencies in place, try to tighten the transformers bolts.
Just a hunch.

[mresistor]

How loud is it?  Bedroom amp?

[TIMBO]

The buzz is only in the main amplifier channel and seems to be generated at the input.


Just bedroom level for sure.

[TIMBO]

A little victory today.
The layout is not great and puts the input grid close to a 120 ohm resistor I have on the PT secondary.
A little metal shield between the grid and resistor has killed the buzz.

[kagliostro]

👍👍👍

I have no doubt about you haved be able solve the problem 😉😁😁😁

Franco

[TIMBO]

Thanks mate,
I'm keen to get into a cab to get a better idea of the sound.


The power valve is getting hammered, so a bit more tweaking....

[TIMBO]

Hi guys, I been waiting for an 8" speaker to turn up to get this into a cab.
Cab has been constructed....


Tried to get a bit of directional stuff happ'n
A nice 8" Peerless alnico speaker to do the reverb thing and 10" alnico noname for the main amp.
Unfortunately both speakers are 8ohm and the OT's are 15ohm..... works great!!

[AmberB]

From looking at the schematic, I can't tell what you did for a ground reference for the heater string.
My suggestion would be to use a 100 ohm resistor from each heater connection of the power tube, V2, pins 4 and 5, and connect the other lead of each of the resistors to the cathode of V2, pin 2.  This biases the heater circuit above ground and usually takes care of any heater related buzz that you might have.

[TIMBO]

Thanks mate, Haven't updated the schem.
 I have paralleled the two sets of cathode resistors and bypass caps on the power valves, and the cathodes are connected to two 100 ohm resistors on each leg of the heater supply.
The amp is relatively quiet with nothing plugged in, with an acceptable amount of hum with the guitar plugged in.

[TIMBO]

Hey guys,
Got some time to get this one moving along

[shaun]

Very interesting build, and looks smart! Also, very good for building muscles when you have to move it ;).

[TIMBO]

Hey guys, This one is done





I replaced the 6BM8 in the main channel with a 6GW8 for a little more power and it also handled the blocking distortion better.
My stunt guitarist gave it a bash and got the nod.
Enough power for a practice/recording and a bit of grind when pushed.
The reverb is not too over the top but is lush and strong with a hint of delay/echo.

[tubenit]

You come up with some incredibly innovative nice builds!  Thanks so much for sharing!  Looks great.


With respect, Tubenit

[TIMBO]

Thanks mate!
I really like the REVERB, I didn't think there could be another way a reverb could sound.
I'm thinking of a PP circuit to drive the tank, maybe a pair of EL84.
Not sure if a TANK would handle the extra power

[glass54]

Hi Timbo,
Nice Work mate. Cabinet is quite novel  and great craftsmanship.
Have you considered using another 6BM8 to drive the Reverb?
Less power than a pair of EL84 ie safer for the TANK. Have seen it on other vintage amps.
Kind regards
Mirek

[TIMBO]

PP 6BM8 is not a bad idea.
SE OT's are expensive. I can get PP OT quite cheap and 6bm8's also

[Platefire]

Wow! It looks like something from the past and modern all at the same time. Sharp Dressed Amp. Great Job!

[tdvt]

Beautiful job!

I couldn't quite wrap my head around the pic of the raw cab, but seeing it covered, with the crease in the side grilles, it all makes sense.

Definitely thinking outside of the box, :)

How did you come up with that design?

[shooter]

he lives on a island 

[glass54]

he lives on a island 

with population of 26 million and he eats vegemite (Aussie Brainfood  )
Regards
Mirek

[TIMBO]

I'm more a HONEY man
Vegemite not a big fan
Peanut paste(butter) sticks to the roof of your mouth

[kagliostro]

For those who can be interested (I'm  )





it is only a bit more readable version of what TIMBO posted

Thanks TIMBO

Franco

[dogburn]

Cool project. The basket of that 10" alnico looks like it would be an Oxford speaker to me.

[TIMBO]

Thanks Franco.
I made a few more changes
Yep can confirm it's an oxford

[kagliostro]

Thanks TIMBO 

Franco

[Auke Jolman]

Hello Timbo,

In the Reverb channel there are 2 electrolytic caps after tapping the signal of the speaker. The polarity of those caps point each in a different direction. Could you explain what is happening there?

[TIMBO]

In the Reverb channel there are 2 electrolytic caps after tapping the signal of the speaker. The polarity of those caps point each in a different direction. Could you explain what is happening there?



Absolutely not, cause I have no idea   

[kagliostro]

That way you obtain a non polarized e-cap

May be a non polarized cap was disposable and someone used it ?

Now a day a non polarized cap that isn't an e-cap may be found also for affordable price, if it was a polyester or so cap we can think also it was used for it's stability on the time, but didn't seems this the reason as it is an e-cap

May be it was necessary to have a high capacitance and a polarized cap was considered not correct in that position ?

TIMBO why did you draw such a component there also if you don't know the reason for such a choice, can you explain ?

Franco

[sluckey]

In the Reverb channel there are 2 electrolytic caps after tapping the signal of the speaker. The polarity of those caps point each in a different direction. Could you explain what is happening there?

The two 22µF E caps connected like that make an equivalent 11µF non-polarized cap.

[kagliostro]

What I would like to know is why those caps are there in that manner

Which is the origin of the choice to put it on the schematic (and the amp I suppose)

TIMBO did you followed a schematic from some source ?

Franco

[TIMBO]

Ampeg super echo twin

[shooter]

you'll find the non-polarized E-caps in many SS audio amps, my guess is it forms a series pass filter, rejecting stuff above the audio range, but, no math today, it's harvest season 

[kagliostro]

Many Thanks TIMBO

Franco

[Auke Jolman]

Thanks for the explanation.

[glass54]

Franco,
Twenty five years or so ago, it was common practice to use a pair of Electrolytic capacitors in series, (-Neg back to back) to achieve a higher value NON POLAR capacitor at half the uF, at cost effective prices. Used it myself many times (this overcame the DC offset of using just one Electro)
As the supply of Non-Polar caps became readily available and Very cost effective, they are now the "Norm" for coupling caps in quality audio.
Especially used in High Quality mic preamps, calibration test gear, etc. I'm talking values such as 4u7 25V thru to 100u 63V. They are used to eliminate the DC offset in normal Electros and are superior to Tantulums (which don't like reverse voltages) and they come in similar size to normal electrolytic's of same value.
Trust this helps.
Kind Regards
Mirek