Latest Bass Amp

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thanks

Postby cw » Tue Nov 20, 2007 12:46 pm

thanks don. it might be awhile but i will proceed.
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Postby cw » Sat Jan 12, 2008 12:23 pm

As I understand it, but please do not think me ungrateful by making some changes.

I am going to try the BFA with a few changes. A bigger plate load on the input 12AX7 as the plate resistance is 62.5K for the 12AX7 which 120K is about twice the plate resistance.
The plate voltage is high; the cathode resistor is low so I think it will drive at too low a gain.
The 6SL7 plate dissipation looks to be too high and it might run out of gas before it drives the 6550’s to full power. A 6SN7, 12AT7, 12BH7, 6CG7 might do the trick. The 6SL7 is fine for a smaller amp. This is not a small amp! The 157Q is designed for a much higher current so I’m going to try a 157G (30H, 40mA). A single 12AX7 or 6SL7 only draws between .75mA and maybe 1.5mA. The total draw on the input and phase splitter should be about 4mA. It should take about 120V PK-PK to drive those 6550’s to full power.

Wish me luck and thanks for the inspiration.
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Postby dhuebert » Sun Jan 13, 2008 9:08 pm

A bigger plate load on the input 12AX7 as the plate resistance is 62.5K for the 12AX7 which 120K is about twice the plate resistance.


If you keep the two input stages the same and switch to 120K plate resistors, you will have too much gain. I would suggest instead to use 12AU7 which is what it was designed for. Or take out the second stage and use 150K plate resistor on the first one, this works as well. The 12AX7 was a mistake that I did not move to correct since people like the amp so much. Also remove the bootstrapping crap on the input and just use a 10meg resistor.


The 6SL7 plate dissipation looks to be too high and it might run out of gas before it drives the 6550’s to full power


I made sure that the 6SL7 would do the job when I prototyped it. The output section drives into clipping before the phase invertor. I have encountered this problem before and so test very carefully before building.
I picked the 6SL7 for two reasons: 1. It is an octal tube and I thought it would look cool and old-timey 2. I like to distribute the gain across a few stages. Supposedly this adds even harmonics without being obnoxious about it and is quieter (shot and thermal noise). So between the large gain of the preamp and the somewhat lesser gain of the 6SL7 I should end up with a powerful good sounding amp.


The 157Q is designed for a much higher current so I’m going to try a 157G (30H, 40mA)


Go for it. (don't forget about the screen grids)

Later: You might want to try playing around with the values in the tone stack with Duncanamps tone stack calculator. I got those values from Garnet and they may merit revisiting.

Don
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Ok Don, Power Supply Questions!

Postby jar240 » Fri Jan 25, 2008 11:05 am

Hey Don, here are some questions about the power supply schematic...(my questions are for learning since I don't really understand exactly what I'm looking at -- I need to study power supplies more!)

1) The Hammond 278CX PT has a center-tap on the heater winding. It looks like you've balanced the heater windings manually using the 100 ohm resistors. Why didn't you use the centre-tap either grounded or biased to a DC voltage instead?

2) Why didn't you Zener-regulate the bias voltage (pre-20K bias pot)? Does regulation matter here?

3) What's bias voltage range you were targeting?

3.5) What's the current draw for the bias voltage?

4) Why 1N5408's instead of 1N4007?

5) The filtering section looks like a giant Pi filter, but I don't know why there are caps in series, and that are parallel with the 100K resistors. Is this some method to allow you to use lower-voltage caps? Maybe you could give me a brief explanation or point to where I might find the answer.

6) The 157Q choke is good for 400V, according to the Hammond site, yet it appears you've got 535V "going into" it. Is this right?

Thanks a lot!

Chris
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Postby dhuebert » Fri Jan 25, 2008 2:38 pm

1) The Hammond 278CX PT has a center-tap on the heater winding. It looks like you've balanced the heater windings manually using the 100 ohm resistors. Why didn't you use the centre-tap either grounded or biased to a DC voltage instead?


I have seen designs biased to a DC voltage and agree this is probably the best solution. It does, however, introduce a level of complexity in terms of having to generate that DC voltage. The way I, and many others, do it is simple and effective. The reason we don't use the center tap is due to irregularities in transformer windings. If the center tap is not exactly center you will have problems.

Why didn't you Zener-regulate the bias voltage (pre-20K bias pot)? Does regulation matter here?


Another level of complexity that isn't really necessary. It's important to remember here that B+ is unregulated; (and shouldn't be) so if the bias supply is tightly regulated the two things kinda work against each other and you wind up with 6 of one and a half dozen of the other. There is a school of thought that bias should rise and fall with B+.

What's the bias voltage range you were targeting?


Just enough.

What's the current draw for the bias voltage?


If I understand you correctly, the bias circuit should be about -50V into 100K=0.5 mA



Why 1N5408's instead of 1N4007?


I don't remember; anyone?

The filtering section looks like a giant Pi filter, but I don't know why there are caps in series, and that are parallel with the 100K resistors. Is this some method to allow you to use lower-voltage caps? Maybe you could give me a brief explanation or point to where I might find the answer


Caps in series=higher voltage, resistors in parallel help balance small differences in value in caps. 600 volt caps will eliminate both. More is more. Tube amp power supplies were something I had a little trouble getting my head around as well. I have looked at hundreds of schematics in an effort to understand what the hell?

The 157Q choke is good for 400V, according to the Hammond site, yet it appears you've got 535V "going into" it. Is this right?


Never saw that, seems OK so far.

It's important to remember that tubes are very bendy stretchy and can survive wide variations in operating conditions. It dosen't pay to be too anal about things, especially with guitar amps. You will notice no global negative feedback in my designs and yet they work well. If you tried that with solid state you wouldn't get very far. Experience has been my best teacher, I learned far more by building than by reading. If you screw up and then sort things out you will have done well, Grasshopper.

Don
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Postby dhuebert » Fri Jan 25, 2008 3:21 pm

Here's how I prototyped my first bass amp BTW:

Image


Don
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Postby jar240 » Fri Jan 25, 2008 3:46 pm

Thanks, all useful information. I'm just thinking that I'll have at least $700 in parts in there, and I really don't want to wreck any of it, or kill myself, or burn the house down! :)

Chris
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Postby dhuebert » Fri Jan 25, 2008 7:41 pm

I really don't want to wreck any of it, or kill myself, or burn the house down!


A very good point. I am often nervous about firing things up for the first time. I wire a circuit and put it down for a day. I then come back to it and check it carefully against the schematic, wire up the next part and put it down again. Pick it up later check it from the beginning and move on to the next part. This way the whole circuit is checked several times by the time it gets power. I have had good success with this method over the years, and yes I often fimd mistakes.

Don
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Postby jlaney » Sat Jan 26, 2008 2:22 pm

Wow, look at that load resistor!

Jim

dhuebert wrote:Here's how I prototyped my first bass amp BTW:
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Postby Shannon Parks » Mon Jan 28, 2008 7:25 am

dhuebert wrote:Here's how I prototyped my first bass amp BTW:


I love this picture.
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Postby EWBrown » Mon Jan 28, 2008 7:30 am

4) Why 1N5408's instead of 1N4007?


1N5408s are 3 Amp rated, 1N4007s are 1 amp. Both 1000V rated.

I call it cheap insurance (bigger is better) Yellow_Light_Colorz_PDT_01 Yellow_Light_Colorz_PDT_03


Another nice rectifier I often use: MUR 4100F (1000 Volt, 4A, fast recovery). I had a bunch of 'em in the parts stash, so why not...

I used these in both of my recent 13EM7 SETs, in a FWB configuration, they're serious overkill, but hey, they work Yellow_Light_Colorz_PDT_03

Your bench setup kinda resembles mine, only yours is a LOT neater and cleaner :o Yellow_Light_Colorz_PDT_02

/ed B in NH
Last edited by EWBrown on Thu Feb 07, 2008 7:12 am, edited 1 time in total.
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Hammond 159M instead of 157Q...

Postby jar240 » Wed Feb 06, 2008 2:01 pm

Hey Don, I have a Hammond 159M, specs:
159M: 15H 100mA 256ohms 500V

The 100mA (as opposed to the 150mA of the 157Q) looks okay for the B+ of the pre and phase splitter tubes, but will I need to adjust the value of the Caps of the PI filter in which the choke is used?

About those filter caps in the PS, particularly int he Pi philter...what voltage rating should they be?

(I've bought the trannies; no turning back now!)

Thanks,
Chris
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Postby Gingertube » Wed Feb 06, 2008 11:05 pm

Guys,
Note those Octal Relay Sockets Don uses in his prototype set up. They are about 1/3 the price of an output tube and provide screw terminal connections rated at 10 Amps to each pin.

A truely great way to prototype power stages using octal tubes - RECOMMENDED. Every hobbiest should own 4 to 6 of these things.

Cheers,
Ian
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Postby Shannon Parks » Thu Feb 07, 2008 6:23 am

Gingertube wrote:Guys,
Note those Octal Relay Sockets Don uses in his prototype set up. They are about 1/3 the price of an output tube and provide screw terminal connections rated at 10 Amps to each pin.

A truely great way to prototype power stages using octal tubes - RECOMMENDED. Every hobbiest should own 4 to 6 of these things.

Cheers,
Ian


My bud Thermion has a jig with several of these along with jumpers and standard resistor values with leaded ends. It's like a electronic Erector set!
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Postby EWBrown » Thu Feb 07, 2008 7:18 am

I look for these octal relay sockets ad flea markets and swapmeets, they're usually 50 ecnts to a buck each, and even though they're well-used, the full-surround pin contacts still grip the tube pins as tightly as that legendary hungry pit bull... Yellow_Light_Colorz_PDT_07


I did screw up once, I found some nice-looking 12-pinners of this type, and I had thought that they would be perfect for compactrons.

BZZZZZZTTTT!!!! Wrong! The number of pins was correct, but the receptacle diameters (too large) and the pin spacing were all wrong... :o Yellow_Light_Colorz_PDT_21
These weren't the usual 11 pin relay sockets, they do indeed have 12 contacts Yellow_Light_Colorz_PDT_01

At least they were only 25 cents each, at the very rainy and cold NEAR-fest last October. Sometimes, the dragon wins...

Oh, yeah, unregulated "fixed" biasing is the way to go, as it will then more or less track the B+ voltage, as line voltage and loading varies, and will maintain better and more stable plate current / biasing conditions.

/ed B in NH
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