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Old 19th April 2011, 01:12 AM   #11
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As for bass response I don't really see anything in the schematic that would really limit bass response all that much. The coupling caps look fine to me. You might want to increase the input cap to say .47uf or even eliminate it all together. Unfortunately I suspect that the best thing you can do for bass response is to choose good output transformer of which, as you note, your choices are limited. Maybe the Hammonds will be pretty good though.

For reliability reasons I don't like the lack of grid leak resistor on the input tube (it uses the volume pot to provide the ground reference) so I would put a 1meg or so resistor from the volume pot wiper (the part that connects to the first tube's grid) to ground. You can do that right at the pot without changing the circuit board (if it uses a circuit board) just be sure that you are going to ground and not the input side. This will keep the tube properly biased even when (not if) the wiper becomes intermittent.

For a safety discharge resistor just put a resistor across C7. I think something like a couple hundred K should be fine. It would only add a milliamp or so in draw and would discharge the caps when turned off. Others with better math recall can calculate the discharge rate and see if small resistor is needed.

I am not sure what specs provided by S5 that you do not understand. Can you be more specific?
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Old 19th April 2011, 01:37 PM   #12
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Thanks mashaffer to chime in again

I decided to do all possible upgrades/mods from the start to eliminate future disassembling of the amps. And perhaps to get the right sound from the start
IF I'M WRONG PLEASE CORRECT ME WHOEVER HAS BETTER KNOWLEDGE, I'm ONLY STARTING IN THIS FIELD
So what I figured out so far:
- OPT's will be Hammond 1609 (i need 9800ohm, original OPT's were 35:1 ratio = 1225x8 (primary impedance)

- PT's not decided yet

Safety:
- bleeder resistors 270k (390k optional?) on filter caps (perhaps 1 or 2 should do, its a cheap mod and i think well worth it)

Performance: (?)
- filter caps C6, C7 and C3 to 220uF, 220uF and 100uF 400V

- 0.01uF cap across HV of power transformer leads (hum?)

- UF4007 on the bridge (is one enough?)

- coupling caps C4 and C5 to better quality (what voltage rating?)

- R9 and R10 to little bit higher Wattage rating to run cooler (I heard they run very hot)

- grid leak resistor on input side 1M carbon film (mashaffer's idea)

I thing thats about it. I'm thinking about the snubber caps as well, but have to read a little bit more on that.

Some parts wont be mounted on the board. These are:
- speaker terminals (on chassis)
- input RCA (on chassis)
- volume pot (on chassis)
- mains connection (on chassis)
- fuse (on chassis)

Some points of concern:
- fuse on chassis - should I remove and jump the fuse connectors on the PCB or just leave it there as double safety? original value is 1.5A, so I'd use for the chassis mounted 2A. Good Idea?
- volume knob - as this will be chassis mounted, I would have to have a wire running from the volume all the way to the PCB - OR - I make a grounding point on the chassis (multiple groundings, don't know how you call it) and lead the ground from mains, PCB, trans etc to that point.

Uff. A lot of things to learn and do. I believe the finished price will be way above 400USD, but the learning is well worth it.

Later will post some pics of my soldering skills

Danny

Edit: i was reading on the filament snubbers (oh damn, the english is confusing ) So according to what I read is to solder 1uF 50V caps across the 3 and 4 pins (heater) of the 6005 tube to eliminate the "ringing" that is if I understand correctly.
Also same caps across the 1 and 9 pins of the 5670 tube.
Am I correct?

THANK YOU IN ADVANCE

Last edited by barackuda; 19th April 2011 at 02:02 PM.
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Old 19th April 2011, 03:19 PM   #13
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picture to previous post about mods...
not sure about the 440uF cap....

Click the image to open in full size.

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Last edited by barackuda; 19th April 2011 at 03:27 PM. Reason: forgot the 0.01uF cap
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Old 20th April 2011, 01:40 AM   #14
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You only need one bleeder resistor. The charge on the other cap will bleed down through R10 anyway. So just one across C6 is fine.

Your idea of going with higher voltage caps for the PS is a good one IMO. Increasing the capacitance on them may or may not be OK. I suspect you would be fine but if you end up having stability problems that would be one place to look.

I am not sure what the point of the cap across the PT secondary is. I don't see how it would reduce hum at all. The only thing I see it doing is reducing high frequency hash. I think that switching noise snubbers are typically placed one across each diode in the bridge. Is the bridge an integrated unit or made up of individual diodes? If you are going to use the UF4007 diodes then all four diodes should be replaced by a UF4007.

For C4 and C5 I suggest either Orange Drop or Polyester Film (like mallory 150) according to taste and what is easiest to mount on your board. These are generally 400V to 600V which is fine. You want the voltage rating to be at least the PS voltage IMO in case of a short at the tube socket.

The bypass cap on the output cathodes won't necessarily improve bass response directly but it would reduce the output impedance (which depending on your speaker) might actually reduce response at the speakers resonance frequency. Still it is probably worth doing. 440uf is however excessively large IMO. 100uf should be plenty I think. I would shoot for 50V if possible (shouldn't be hard to find) again to give a bit of margin.

I am not sure the purpose of the cap across the heaters. Again it would seem to be for high frequency noise reduction which I have not really heard of being a problem. What I would do if possible is to connect the center tap of the heater winding to the top of R9 instead of to ground. This will bias up the heater (about 15V I would guess). This will help prevent heater hum getting in to the cathode of (especially) the first gain stage. Zero cost option to guard against hum.

If moving the fuse to the chassis then I would eliminate the on board fuse and use the exact same rating and type (e.g. slow blow or fast blow) as the original.

Use shielded wire for the connection between the volume pot, jacks and PCB. As much as possible you want to have only one grounding point and not multiple points on chassis and PCB. Also you don't want both ends of a cable shield grounded. Multiple grounding points can lead to ground loops (current running through the ground) contributing to hum problems.

Hope I touched on everything. I will have to do some reading on the heater ringing thing. That is new to me.
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Last edited by mashaffer; 20th April 2011 at 01:44 AM.
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Old 20th April 2011, 08:00 AM   #15
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Hello Mike,
well you are certainly very helpful...as you are the only one to reply
All these mods come from people who have build the 8LS (stereo version of this amp) or the K family of amps. The 8LM is basically half of the 8LS and the K family is almost the same circuitry. So I put together the most used mods.

For the cap across the HV of PT, it should reduce high frequency noise (fuzzy) according to Voltseconds page. And a second thing as a plus is to reduce (not eliminate) the pop coming from the speakers when you turn of the amp.
I also read elsewhere that the snubbers should clean up the signal and are a good thing to do (its quite cheap so why not).
He said that after installing the snubber caps the tweeter noise was 100% gone and it reduced the bass/midrange hum.

I have found a good shop for "hi-end" parts (and I mean really really hi-end Duelund caps for 3000 :O ) in UK. They also carry some more usual stuff like pots, tubes, resistors, other caps, etc.
here the link for EU members
amplifier valve kits, HIFI pre-amplifiers, speaker kits,AMP Parts, upgrade components

Actually for the C4 C5 caps I think something in the range of 8-10 euros for cap is a good one quality wise. I'm not spending ridiculous money on this amp, perhaps on the next one for the living room

As for the center tap of the heater connecting to the R9 - well I'm lost here what is its purpose? And if I'm correct you mean the YL ?(yellow) cable which goes to ground should go to the R9....? Right?

As for the cable shield grounding, do you mean that if I for example I take a cable from the RCA socket to the PCB, I should only connect the shield to the ground on one side? Closest to the grounding point? Never heard of that before (but I'm always glad to learn something new).
I want to make one grounding point somewhere on the chassis where I connect the ground from PCB, PT's, pots, RCA etc. Some compromise in distance from all points.
Is this a good idea?

Thanks a lot. I go home now for Easter holiday and probably do some shopping
This tubes are infecting pretty quickly
I can see my self with multiple amps in the future (and perhaps no wife? )

Danny

Last edited by barackuda; 20th April 2011 at 08:14 AM. Reason: Hehehehe my initial price for the caps was a little too low :D
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Old 21st April 2011, 02:37 AM   #16
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I figured that those caps on the heater and PT were for high frequency noise. Off the top of my head I don't think that they should cause any problems. From what I have read I suspect that ceramics would be appropriate there. You might want to consider Y rated safety caps for those as (IIRC) they are designed to fail open so that you wouldn't end up frying your transformer.

If you want to try the high end caps for coupling go for it. You might want to go ahead and purchase some of the orange drops and mallorys as well since they are cheap and that would give you a chance to try them all and let your ears decide what you like best. Cheap educational opportunity me thinks.

Quote:
As for the cable shield grounding, do you mean that if I for example I take a cable from the RCA socket to the PCB, I should only connect the shield to the ground on one side?
Correct. In fact at one end the shield should not be connected to anything. So for example the shield could be connected to the collar of the input jack but left unconnected at the PCB. Then connect the collar of the jack directly to the star grounding point via a length of hookup wire. The inside conductor (center pin on the jack) is connected at both ends of course but the shield is connected only on one end.

On a guitar amp that I did I had two ground points to the chassis. One was right near the input jacks and all grounds after the power tube stage went directly there in a star pattern. The other was near the power transformer and all of the initial PS filters and the power tube stage went here. That two star system worked well but I am coming to the conclusion that a single chassis connection would be better. On my current project I intend to run all grounds to a single point at the can cap (PS filter).

Quote:
As for the center tap of the heater connecting to the R9 - well I'm lost here what is its purpose? And if I'm correct you mean the YL ?(yellow) cable which goes to ground should go to the R9....? Right?
Yes, connecting the center tap that is normally grounded to a voltage higher than the cathode reverse biases a parasitic diode created between the heater and cathode which prevents ac hum leakage into the cathode circuit of the tube. See here for an explanation under the reducing heater hum subsection about 2/3 of the way down...

http://www.freewebs.com/valvewizard/heater.html

You can bias up the heaters with a voltage divider on the B+ supply which allows you to set it exactly where you want it but "stealing" the power tube cathode voltage (which in this case should be about 15V I think) is easier and doesn't require as much extra work on your part. By doing a voltage divider you could get it above the PI cathode too but I don't think that is really necessary.

Have a great Easter.
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Last edited by mashaffer; 21st April 2011 at 02:41 AM.
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