Hi-Gain SE

[angelodp]

Hi, I have been experimenting with a type of amp that was used in a Moviola editing systems. Its basically a Champ type amp. It has the both PT and OT that will work for a Champ type build. It also has room for two 12ax7 tubes and a 6V6 and a 5Y3 as rectifier. I have made a Vibro-Champ with one chassis and it works very well. I have also made more of a point to point build that is 'offbeat' at best. Here is a cleaned up layout of the augmented schematic that I worked out. Four gain stages are too much so I have backed off to lower gain tubes and used a voltage divider as well. This amp is strangely interesting in that it has a Vol, Master Vol and Gain as well as the Merlin style tone stack. I can dial in lots oaf different tones, some quite good. You can see a Raw knob that I have not implemented yet. The question is how to balance the 4 stages so that I can get back to 12ax7's and have roughly the same configuration with full control over the various Vol, M Vol and gain controls. Thunderfunk has a four stage design I am looking at but have not heard much about. There is also Swamp Thing that Mark turned me on to.

Thanks Dave for the help on this.

[IMG985]http://i260.photobucket.com/albums/ii9/angelodp1/1801amp-lnoloads-switchV-2_zps85189830.jpg[/img]
[IMG818]http://i260.photobucket.com/albums/ii9/angelodp1/565-amp-layoutcopy-2_zpsea478644.jpg[/img]

[Colossal]

Ange,

You show plate loads for V1a and b at 100k and 110k respectively, but the tube shown is a 12AU7. If you are trying to lower gain, you should consider lowering the plate loads. We spoke about this before so I'm guessing the 100k and 110k values are left over from when V1 was a 12AX7. You may need to change the cathode values as well depending on where you want the tube to idle and what your design goals are i.e. center, cold, or warm biased for each stage.

[tubeswell]

What about using: V1a hot-bias, V1b cold-bias, V2a hot-bias and V2b very-cold bias (with say an unbypassed 39k cathode resistor)?

And beefing up the gain and inter-stage attenuation?

[angelodp]

Dave, I have been swapping tubes to try and tame the gain. V1- is 12AT7 Gain factor - 60
V2 - is 12 AY7 gain factor - 45

This helped get the amp into useable territory. On the Plate loads - I like the alternating bias idea. The design goal is to have the amp top out with good sustain and hi-gain while running 12ax7's in both positions.

Ange,

You show plate loads for V1a and b at 100k and 110k respectively, but the tube shown is a 12AU7. If you are trying to lower gain, you should consider lowering the plate loads. We spoke about this before so I'm guessing the 100k and 110k values are left over from when V1 was a 12AX7. You may need to change the cathode values as well depending on where you want the tube to idle and what your design goals are i.e. center, cold, or warm biased for each stage.

[sepulchre]

You might consider some local negative feedback to tame some of the stages.

[matt h]

(deleted)

[sepulchre]

[roberto]

You can simply use splitted plate loads to dampen the gain of the 12ax7, and keep everything else as is.

[angelodp]

Roberto, can you elaborate on how that split plate load would work out.

A

[Leo_Gnardo]

Roberto, can you elaborate on how that split plate load would work out.

A

I'll volunteer an answer 'cause Roberto is likely enjoying a well deserved night's sleep.

Use two series resistors instead of a single one in your pre plate circuit. Pick the signal off the junction of the R's; put your DC blocking cap to the next stage there. You can decide how much signal attenuation by selecting resistor values; they act as a voltage divider for your amplified signal. The sum should be a normal plate R value like 100K to 220K. Fender used this technique in some early 60's brown/white amps.

[matt h]

(deleted)

[angelodp]

Let me look around a bit for that idea. I have a Brown amp here. If I am not mistaken the Encore has an arrangement similar in nature. So you are actually talking about cutting the AC signal at the grid, tailoring it to suit.

So the beauty of a four gain stage is adding rich overlays of frequency and harmonics without blowing the whole thing up.

Thanks

[Meat&Beer]

Late to the party as usual. You find something to be able to understand what the hell these guys are talking about yet?

Sounds like the Fast/Gradual on a Wreck, is that the Komet that does this? Anyway, this is what it looks like. Say your plate R is 100k, both the plate R's in this would add up to roughly the same. So it'd go, B+, 82K, 22k, plate. Or thereabouts. Caps to taste.

I personally like 12AU7's a bunch.

[angelodp]

Thats cool, so the Komet has this setup? Any others. Is that schematic avail.

[roberto]

I'll volunteer an answer 'cause Roberto is likely enjoying a well deserved night's sleep.

Thank you Leo!