100w bASS to GUITAR CONVERSION

[TIMBO]

Hi guys, Got a local built 100w bass amp that needs to be converted.
It is not working at the moment so I can't hear what it sounds like with a guitar plugged in.
So I thought I'd just pull the boards anyway cause a couple of the Ecaps are stuffed and the preamp values are more suited for bass (cause it's a bass amp  )
And some darkening of the circuit board around 220k resistors at the top right of the top board.
Seems to have had a small fire here at sometime. Resistors may have been replaced.

Transformers checked out OK.
Did a calculation on the OT and look to have a 3K primary and 4,8 and 15 secondary
Can't find any pictures that can confirm power valves,i would think EL34/ 6550
PT 6.3v CT and 320v HT
Schematic traced.
Looks to be fenderish up front and marshall major in the engine room.

My thoughts are............
Changing the voltage doubler to bridge or half wave rectifier.
Converting the two channels to something more interesting 
Changing some of the coupling caps in the power amp stage to be more guitar friendly.
Removing the string of resistors from the power valve plates (a new one to me)
Possibly remove two power valves and rewire OT for 50w output. (2x EL34)
Or two 6550's
Maybe even use a quad 6DQ6's as they sound awesome

[TIMBO]

A couple more pics

[trobbins]

PDF schematic available?

[kagliostro]

Here is the content of the BAEZ 100W Amp.sch file







----

........... string of resistors from the power valve plates (a new one to me)

Are you talking about resistors to prevent oscillations placed as plate resistor of the power tubes ?

http://el34world.com/Forum/index.php?topic=20857.msg220596#msg220596

read Reply #13

Franco

[sluckey]

Here's the entire schematic in pdf format...

[TIMBO]

Thanks guys, Being a fender up front and room on the PCB to add extra componets will make converting Channel 1 to a more guitar friendly circuit easy. I'll leave Channel 2 as is , might make blending the two channels interesting.
I'll rewire the Bias supply similar to a marshall type.
Should be easy enough the do a bridge rectifier and drop the HT voltage.
Calculated the OT primary as 3K so removing two output valves will work with a few valves.
Plugging 15 into 8 and 8 into 4 will also give a few options.

[TIMBO]

Seems I not as smart as I thought 
There was a reason that the original bias is done as per the original drawing.
"Capacitive Coupled DC bias supply.
I know that this was discussed at some time.

[kagliostro]

Ciao TIMBO

as you are on the mod way why don't adopt the JCM900 bias circuit style that is wiper fail-proof ?



Franco

[TIMBO]

Thanks K, Just had a look at valve wizard site and got some ideas.
That looks even better.

[kagliostro]

From same author






8.1 is the same you can find on the web page, 8.4 is a version with balance for two power tubes (see faint part of schematic)

Franco

[sluckey]

Here's another bit about that bias supply. Look at pages 5 and 6...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[TIMBO]

Thanks guys, Rewired supply and i'll have to get a class X cap.
I have drawn it a bit different, that cause it is setout that way on the board.
The Class X cap are rated at 250v will that be enough???


I have a 56k in series with 22k trimpot, that will change cause I been told those trimpots are trouble.

[TIMBO]

Hi guys, bias supply up and running.
The local electronics shop didn't have and .047u class X caps, so I paralleled two.022u, this seems to work fine, good bias range -27 to-43, this should cover most valves in the 50w range.
Wired in the HT and unloaded is around 450v.

I now have to decide on what output valves to use 
As is, all impedances reflect a 3k primary
15 into 8 - 5.6k
8 into 4 - 6.4k

[kagliostro]

I know it may be odd

but a pair of 807 Class AB2 with 400V B+, 300V G2, -25V Bias, 240mA Plate + 23mA Screen

gives 55W on a 3200ohm load

(I don't know if you can do the same with 6L6 tubes)

Franco

[PRR]

Class AB2 will need a very different driver.

I think EL34 will drop in and work fine. Marshall often aimed for 3.2K/pair, no real difference.

[kagliostro]

As always PRR know the right way

 

on the EL34 datasheet you can find a 44W @ 350v B+ with 2.800ohm load and a 55W @ 400v B+ with 3.400ohm load

Franco

[TIMBO]

Hi guys, got another head scratcher 
I just found the heater supply for the four power valves is 25.2v NO CT (27v unloaded) so the four 6550's were wired in series.
Now that I'm dropping two valves and likely to use two EL34s I need the reduce the heater voltage.
I have two ideas.....
Idea 2, is to do something similar to the 12.6v supply where I had a single valve I couldn't pair with to get the 12.6v, a resistor was calculated and put in series with the valve.
So could two resistors be used the replace the two removed valves heaters.

Idea 3, is to use to 240v-30v transformers in parallel (240v-60v) to get a dropping ratio of 4:1 thus dropping the 25.2v to approx.6.3v
The only transformers I can get are 1A each.

[kagliostro]

Ciao TIMBO

I don't like much the transformer option, if I had to choose, I'll prefer resistors

and ... 480/60 = 8 ... not 4

to have 4:1 you must connect the 240V windings in parallel and the 30V windings in series (240/60 = 4)

---

Have you a 240V/120V transformer ? this way 27v/2 = 13.5v and you can connect the tubes in series at 1.5A consumption instead of 6.3v at 3A

Franco

[darryl]

If you're already considering an extra transformer, wire the EL34 heaters in series and use a stock transformer with a 12-volt tap, for example: https://www.jaycar.com.au/6-15v-30va-2a-multi-tapped-transformer/p/MM2004

[kagliostro]

That is the egg of Columbus 



+ 1 for Darryl (if the transformer is your choice)

Franco

[PRR]

Losing 12V at 1.6A is _20_Watts_ of waste heat to be handled.

You can put four EL34 in a 50 Watt amp. Tube life may be extended.

OTOH, getting a 6V supply means more reliability in the off chance a heater breaks. Series, ALL tubes quit. Parallel means the amp will limp along.

[kagliostro]

Series, ALL tubes quit

yes, but in a PP there is not much difference, may be things go better in a PPP

or I'm wrong ?

Franco

[TIMBO]

Thanks Darryl, works a treat.

[TIMBO]

Got the last of the wiring done this afternoon and injected some life into the old girl and all's good.
I originally had 12AX7 in V3 and V4, this seemed to cause some blocking distortion.
Darryl had suggested that 12AU7 would possibly be better in these positions.
Bit late to get stuck into it but at low level sounded fine.

[TIMBO]

Hi guys, With some advice from Darryl V3 was returned to a 12AX7 and I think the amp sounds great.
Trying channel 2, I was surprised how bright it is and also sounds great.
Jumpering the two channels thickens the sound, Awesome!!!




Cab needs some attention.

[TIMBO]

Hi guys, Cabs done, voltages checked sounds great, done.