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PostPosted: 07 Nov 2011, 15:03 
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Joined: 14 Oct 2008, 17:35
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Location: Dartmouth, Nova Scotia
You got quite preserved amplifier there, I will have to make up the modified schematic, I think you should keep the origianl design like I said, it is very easy to add effect such as over drive in the preamp section, (one more thing that I plan on doing).

I'm not using Mullard tubes for my amplifier, they are mixed Sylvania and Philips ones, and yes, you can over drive this amplifier quite easily, I cranked the volume up really high and plucked the E string on my base and the cheap 12" speaker I had the amp hooked up to almost rattled to pieces, the plates of the output tubes got a bit orange, but no failure.

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PostPosted: 08 Nov 2011, 03:55 
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Joined: 03 Nov 2011, 04:54
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Actually there is some oxidation and rust. I tried to clean the fuse holder and the transformer's selectors but I'm not sure they will make good contact. I saw that you have replaced your fuse holder - I have to do that too.

Are you sure your Sylvanias are not rebranded Mullards? They look to me like xF2 with double getters. Mine are the same but three are labeled Philips and one - Dario. Check for Philips codes near the tube bases. Your Philips tube is made in Holland or Belgium, I think.

How do you biased the output tubes? On one of the pictures I see 817V - maybe this is the anode voltage but did you measured the currents in idle?
It will be interesting to see your schematic. What is the purpose of the three LED's?


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PostPosted: 17 Nov 2011, 17:37 
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Here are the schematics I have made... I have a modified one of the amplifier that has an LED biased inverter stage and Ni-Cd battery biased input stage, and I think a few other changes to the original circuit, still have to experiment with them, but it sounds really good. :D

pch64 wrote:
How do you biased the output tubes? On one of the pictures I see 817V - maybe this is the anode voltage but did you measured the currents in idle?
It will be interesting to see your schematic. What is the purpose of the three LED's?


I bias the output tubes using a regulated negative bias supply using a mosfet and a voltage divider with a variable resistor and a fixed resistor. This supply is much more reliable than the old one that just uses a germanium diode, filter cap, and a voltage divider to give a fixed un-adjustable voltage. :thumbsup:

The 817 volts you see is the anode voltage going to the tubes and the output transformer, the LED's are just some fun I added :) Two of the LED's indicate when the 820V B+ is activated or in standby, two others indicate the Anode supply relay status (active or stand by), one LED indicates the anode supply for the inverter/driver and preamp stage is active, and the final one indicates the bias supply is active.

pch64 wrote:
Are you sure your Sylvanias are not rebranded Mullards? They look to me like xF2 with double getters. Mine are the same but three are labeled Philips and one - Dario. Check for Philips codes near the tube bases. Your Philips tube is made in Holland or Belgium, I think.


They could be, but not sure how to tell the difference, but, they aren't dead yet, and they don't look like the tubes in that amp that had a nuclear melt down. I am going to look into some overload protection though as stable as the amplifier is, you can still heat the tubes up quite easily if you drive the beast too hard. :hot:


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PostPosted: 21 Nov 2011, 06:20 
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Thank you for the schematic.
Yesterday I started with removing the connectors on the back of the amp and installing a piece of GPO-3. I also installed an IEC connector on this plate but didn't have time to test the power supply.

I did some calculations and can't understand why they recommend connecting an 8 Ohm speaker to the 10V tap (the Ra-a will be 53 KOhm). If we connect 83 Ohm load at the 100V tap, Ra-a will be 5.7 KOhm which seems to be the optimal load for this amp. There is an example of similiar amp in the Philips datasheets and for two tubes Ra-a is 11 KOhm (Ua = 775V, Ia = 2x25mA, P = 100W).

If your tubes are Mullards they will have similar codes like this one:

Image

Isn't there enough sag in the power supply to keep us from exceeding the maximum dissipation at the full power? According to the manual Ua must drop from 805-860V to 660-720V at full load.

Peter


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PostPosted: 21 Nov 2011, 16:34 
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pch64 wrote:
Isn't there enough sag in the power supply to keep us from exceeding the maximum dissipation at the full power? According to the manual Ua must drop from 805-860V to 660-720V at full load.


What you have to remember is that this information is taken from the amplifier with the original "stock" parts in it. The changes I made to the power supply, with just the rectifiers alone mean that there is much more rectified voltage. Also the larger caps will provide for better filtering and keep a much tighter voltage with better regulation when with the amplifier is fully loaded down.

I did a few calculations in terms of the power supply so as to give an idea of my output voltage with the silicon rectifiers, there is something like 1-2 ohms of resistance between the contacts of the selenium, very high.

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PostPosted: 15 Jan 2013, 19:53 
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Hello all! Been a while but the amplifier still works pretty good, I have been using it here and there and I must say it sounds great for playing bass, but a couple of problems have appeared, one to me seems quite severe and the other is very minor in comparison.

That being said I could use some help in trouble shooting and tracking down the power supply noise. :)

The noise sounds like 120hz buzz more than a 60hz buzz and can be heard from the power transformer and increases in level when the B+ is active as the transformer is now loaded down. The same noise can be heard through the speaker, not loud but loud enough to be bothersome.

The part that really bothers me is that it has always been there and after changing every capacitor in the amplifier and much of the wiring and other parts... no change.....

Ok.... well then.... I was just about to finish saying that maybe my power transformer was causing the problem and lone behold! My friend calls me and when I got off the phone with him.... I realize that I don't hear the amplifier playing music anymore... as I go down in the basement room, I smell something dying.... :blush:

Yep, should have known better because I think the power tranny is toast now :( The primary fuse has blown and one side of the insulation has a bubble on it and is burning hot! :hot:

Btw, Peter how is your restoration going? have any luck?

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PostPosted: 23 Jun 2013, 20:38 
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Looking back at the old picks amaze me compared to how the amp looks now...anyways after much frustration an empty wallet and time consuming efforts I brought the amp back to life and now it runs perfect....well almost.... the bees in the amp never left... so I'm open for suggestion on why that could be.

For now, here is a schematic update!


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PostPosted: 19 Jul 2015, 08:19 
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Joined: 19 Jul 2015, 06:15
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Location: Iquique, Chile
Hello everyone!
resurrecting this thread, I'm sort of new in the forum and mid-level in amp modding(reading schematics, recognize/replace components, probe troubleshoot, etc), done quite a few mods to this amp, but it's clear I don't seem to get it to work because of many issues with connectors, wiring and stages...trying to re-wire the back panel with new inputs, just like u did with the one in the pictures.
I've been thinking about the relay mod for the OP tubes since it's very effective for what it's intended, for me, I'm into turning this into a bass head too for medium size-regular gigs and rehearsals in the outside, backyard, maybe squares and assorted gigs in the hood, the thing is that I bought this amp already modded, with few mods on it, just the mic inputs turned into guitar jack cutting the path from the back and drilling holes for the jacks on the front chassis where the pots rest, a mono preamp was installed, I guess by the previous owner or some tech from my country, cause it's a spanish labeled PCB, but it didn't feel that useful to my taste so I removed and I'm planning on etching some other preamp, maybe a SVT preamp stage or the Bassdriver DI or a preamp/compressor altogether, just to get rid of the drilled holes and the remaining slot between the head cab and the bottom plate where the seleniums are laying. I bought it outside my city, so It was kinda surprise to buy it as it was, but i have to admit it looks like charm, It has a nice tolex cover too, aquamarine-coloured made for such a robust amp like this one and comfortable to handle, but way too heavy to carry it a few blocks (haha)
Got this for about 60bucks in my country(chile) and since I bought it, I've been dying to mod it the way it deserves and letting a good preamp for bass and maybe a microphone input so I can mix channels since I play in a band where I gotta sing over bass lines more than just playing, but it's not mandatory by now, so I was asking for any advice about this amp, care tips for the transformers since they're like too vintage to mess with them and at least on the tubes, well, they're blank maybe cause the paint faded by the heat, I forgot to mention, tubes were mis-biased or unmatched for what I saw, it had both 820ohm resistors looking like burnt wood, so those were replaced but I'm still on that for adjustable bias pot and stuff kinda sorting out a few fixes I've already got handy with, while others remain on top of my "to-do list".
I haven't tested it in a good loudspeaker set, so I haven't tested it decently, just tested it with some wharfedale PA speakers when I bought it, and those are far from sounding ok on any bass amp for my taste, also, happened that thing about the amp rattling to pieces, so the new task for when I get some serious cash is to buy at least one 15' spkr and wood to build a cab and rumble the scene!
My guitar player hits the mix with a 100w marshall tube-stack, so I get pretty short on rehearsals and gigs and lost in the mix while beating my SS orange combo (50W crush). The main reason why I'm saving cash for the spkrs is because of that, and also because I need some serious gear at this time.
To get an idea, How is it sound like? sounds like any bass or guitar amp available on the market? I know it might depend on the speakers, instrument, etc, but is there a place for comparison...? I was thinking if it was charged with a quartet of EL34 it might sound british-ish I guess, maybe a marshall or orange.
A dream far away tells to mod it to sunn specs for the kind of sound I'm chasing into (doom/stoner/sludge/drone and some random punk stuff)
Well, Hoping 4 u guys to be still around the forum, give me some advice to get my hands on this.
Greetings from chile, mate!


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