PLEXI + HOTSWITCH

[rzenc]

I would like to build it... Actually I already have a 30W, 6L6 pair. However, this time I'm planning a 6550 quad for the output...and this is my question: I need to custom order PT, OT and choke for this beast...Buying from anyone overseas is out of question since taxes will be awful high...

I checked GE data sheet but as you may know there are no 6550 data sheet in pentode mode with G2 running close to plate voltages (also checked MO-KT88 but no joy either).
Found some specs regarding hammond iron:

1650
Audio Watts (RMS): 120
Primary Impedance (Ohms): 1,900 CT
Maximum DC per side (mA): 403
Secondary Impedance (Ohms): 4/8/16
Mounting centers: 3" x 3.31", vertical mount
Weight: 14.0 lbs.
Suggested tube types: 6L6GC, 5881, EL34, 6550B, KT88 (push-pull parallel, 4 or 6 tubes)

I'm thinking out loud here:

PT1: HT: 375 - 0 - 375 @ 600mA's/ BIAS 60 - 0 - 60 @ 100mA's;
PT2: FIL: 3,15 - 0 - 3,15 @ 8A's;
CHOKE: 10H/100R @ 200mA's;
OT: 1750R PRIMARY / 0 - 4 - 8 - 16 SECONDARY

NOTE: PT1 and PT2 are separete transformers.

Do you think it is suitable?

suicide note: Neighbours give up on me a long time ago..

Thanks in advance!
Best Regards

Rzenc

[rzenc]

I decided to add a FX loop to it. Never build such a loop but will try it.
If you guys can help me check it out 

This is beast will soon be built!

[TIMBO]

Hi rzenc,All looks good. I can't offer much advice on weather it will work but i'm sure it will (and the neighbours will love you for it). Question,what is the thinking behind all the B+ filtering as most builds i do the max i have in the first stages would only be 100uf and having that much what effect does it have on sound/tone?? Hope it works well for you 

[rzenc]

Hi TIMBO!

The reservoir caps equals to 200uF, 4x100uF stacked on top of 4x100uF. I will use e-caps and it's rated at 450V. Expected unload B+ will be around 520V. So there is margin for safety.
I have built some JCM 100W for metal players and they complained about too much saggy when cranking the amps. Adding more filtering at reservoir and screen node helped to tight the amps, improved bass response.
P.I. node on marshall drawings calls for 100uF. Just followed it.
The rest of the amps is pretty stock (plexi+hotswitch).
I would like the FX loop to be as noise free as it can get. Thought that 100uF could provide a clean DC.

I have exactly the same preamp except for the FX loop.
The amp works with 6L6/EL34 quite well pumping around 30W at 380V . But when I plug in 6550's, it sags too much. I need a 100W beast to punish neighbors!(666)!!! It will also be used on shows by a friend which plays large venues. The other guitarrist uses a SLO 100W.
I hope they stop fighting to turn the amp down. 
He wants me to build something to him, but he is not sure about what he wants. So will be playing with it until filament light shine upon him.

Best Regards
Rzenc

[tubenit]

Is this a way that you're trying to offer parallel vs. series FX loop?  The 330uf on the FX return cathode seems rather large?  Not saying it won't work, just saying I've never seen one that large there?

See schematic area highlighted in red.

Can you share where you got the FX loop design, please?

In the ARCHIVES, you can find some FX loops that may also be worth considering or at least comparing to.

http://www.el34world.com/Forum/index.php?topic=10208.0


With respect, Tubenit

[rzenc]

Hi Jeff! Thanks for checking it out.

Can you share where you got the FX loop design, please?

DESIGNING VALVE PREAMPS - M.BLENCOWE, 2009, PG.273, FIG.11.21

There are some minor mods and design differences, but it's roots are from this circuit.

Is this a way that you're trying to offer parallel vs. series FX loop?

Yes. SW04 switches dry signal on/off whilst SW03 switches phase 180°. Always interesting to hear how effects are affected when phase is inverted.


The 330uf on the FX return cathode seems rather large?  Not saying it won't work, just saying I've never seen one that large there?

V3b receives dry signal and fx return signal. They get mixed thru virtual earth pot - RV08. I believe it is a suitable place for noise pick up. Remembering classic designs like 5F6 and JTM45, both employ an unusual large cap on guitar input tube cathode to avoid hum pick-up by sensitive grid circuit. Merlin uses a 100uF in this position. I don't have any 100uF @ 25V but many 330uF and 470uF @ 10V and 25V. Maybe adding 470nF in parallel helps with transient response and since poly caps shows miliohms on ESR may also improve treble response as well.

In the ARCHIVES, you can find some FX loops that may also be worth considering or at least comparing to.

I will check them again. How do you think the dumbleator reacts when a lot of preamp distortion hits it?

Thanks in advance
Best Regards
Rzenc

[tubenit]

How do you think the dumbleator reacts when a lot of preamp distortion hits it?

I think they work great in a Dumblish clone & I think they play a major role in "that" tone.  

Remembering classic designs like 5F6 and JTM45, both employ an unusual large cap on guitar input tube cathode to avoid hum pick-up by sensitive grid circuit.

I thought the 5F6 Bassman used the large value cathode cap  because it was a bass guitar amp?  My understanding is the JTM45 was simply Jim Marshall's version of the 5F6?   As I understand it, having a cathode cap value larger than 25uf is not needed with guitar?  Note that the Dumble amps use much smaller 5uf and have remarkable bass response, IMO.

Again, I am not saying the 330uf won't work.  You can try it and if it's not what you want, then change it to something like 5-22uf instead.

I quite using 25uf cathode caps on 12AX7 because they sound muddy to me in comparison with smaller values like 2.2uf - 10uf. Just a preference thing.

with respect, Tubenit

[rzenc]

I think they work great in a Dumblish clone & I think they play a major role in "that" tone. 

I built 3 or 4 D-styles but none with FX loop. Will build another soon and plan to include it as well.

I thought the 5F6 Bassman used the large value cathode cap  because it was a bass guitar amp?

Ok. It was intended to play bass guitar but the ultra large cathode cap also helped to fight noise pick-up by the most sensitive stage inside the amp.

My understanding is the JTM45 was simply Jim Marshall's version of the 5F6?

Agreed. As said by Pitmann: A way to offer a cheaper bass amp since importing american fenders were prohibitively expensive back in the day.

Note that the Dumble amps use much smaller 5uf and have remarkable bass response, IMO.

I couldn't agree more. One of the D's I built uses KT88's and bass is freaking awesome.
I also agree with you that a tighter bass response by means of cathode bypassing is a smart way to keep bass freqs in check. Instead of employing coupling caps to shave bass, I prefer using cathode bypass to set it. I heard strange things when shaving a lot of bass with coupling caps. But it may just be me.


I quite using 25uf cathode caps on 12AX7 because they sound muddy to me in comparison with smaller values like 2.2uf - 10uf. Just a preference thing.

I was tweaking an amp built for a friend and he was complaning about lack of warmth. So I asked him to explaing me what warmth meant to him. Long story short, I installed a 47uF on V1b cathode - recovery from tonestack - and a smile happened on his face. It was paralleled with 1uF poly - which was the original cathode cap. 

I will take your word for it and keep an eye at that point. Sure it will need tweaking.

The points you guys have showed are all good catches.Thanks a lot men!!

Jeff: Why you asked about the switching series/parallel? Is it too strange?


With Respect
Best Regards
Rzenc

[tubenit]

Why you asked about the switching series/parallel? Is it too strange?

No, not at all. I think it's a good idea. I did that on my FX/Reverb unit that I built. I found I preferred series  most of the time, but liked both tones.

I simply had not seen it drawn that way before.

I think you have thought thru this design well. Please keep us posted on how the build goes.

With respect, Tubenit

[rzenc]

I have almost all parts here, except for irons, pots and 6550's...

I will order Irons today...Here is what I plan to use:

PT01:
HT: 375 - 0 - 375 @ 800mA = 420VA
BIAS: 60 - 0 - 60 @ 100mA's = 8.4VA
PT02:
FIL: 3.15 - 0 - 3.15 @ 8A = 50VA
CHOKE:
10H/100R @ 200mA's;
OT:
1700R PRIMARY / 0 - 4 - 8 - 16 SECONDARY

If you guys see something wrong with iron specs, please shoot!!

Thanks guys!!
Best Regards
Rzenc