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Old 19th April 2006, 05:38 PM   #1
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Default Help with guitar tube amp mods

Hi
I recently bought an old tube amp head which I plan to modify to reduce hum and noise, and turn one of its two clean channels into a ďdirtyĒ one. Iíve built a couple of tube preamps before, but this will be my first project involving power tubes, so please HELP with advice and/or opinion.

Iím not sure of the ampís model and I havenít completely traced the schematic yet. The logo on the chassis reads ďDinakordĒ which is quite of a misspelling of the European company Dynacord . The amp however does not appear to be a DIY because it looks like a factory job Ė it has a professionally made box and chassis, itís laid on a turret board, the transformers are huge and have nice solid covers, etcÖ but who knows!?! The preamp section has two independent clean channels with separate inputs. One channel has two gain stages with an ECC83. The other channelís first stage is half of an ECC83, the second stage is an EF86 which is also a vibrato driver, and the other half of the ECC83 is the vibrato oscillator. Both channels connect together and go (through a 0,01uF cap) into the input of the phase inverter Ėone ECC83. The preamp sectionís grounding scheme uses copper wires running from the turret board to a copper plane which is held to the chassis by the pots. However, some of the preamp grounds, and most other grounds are connected to the chassis or other places. The PA section uses two EL34ís which are biased with a pot into 50V DC made from a separate winding of the PT. The mods I plan to do are: get rid of the vibrato and the EF86 on channel 2, use the EF86 socket for a third ECC83 for the ďto becomeĒ dirty channel, use one half of ECC83 as first stage for both channels, add DC bias of the heating, improve the PS filtering, make some sort of channel switching, and star ground all sections. Here are some questions I have, bearing in mind that my main goal is to reduce as much as possible any hum and noise:

1. The amp has choke which is used only for B+ going to the OT center tap. I have no idea why the B+ for the tubes is taken before the choke instead after. Maybe itís a question of the chokeís rating or maybe this is not a choke at all(!?!). Should I experiment and take the B+ for the amp tubes, or at least B+ for the preamp and driver tubes, after the choke, so they can benefit from its filtering capabilities?

2. Is there a limit to the value of PS filter caps? To reduce noise, I plan to use as large caps as practicable, e.g. two caps 470uF/450V - one before, the other after the choke, and five or six 220uF/450V for the driver and preamp stages, decoupled with resistors. Iím not sure whether using too large caps has any drawbacks..

3. Is there anything wrong with placing PS filter caps inside the chassis? I have enough space and this would facilitate the ďstar groundingĒ without having to drill 6-7 holes in the chassis, but I donít know if the caps would somehow affect the rest of the parts inside.

4. Can I use the existing 50V/DC, which is currently used for biasing the EL34ís, also for DC bias of the tube heating or should I make a voltage divider and use the B+?

5. Is it a good idea to use DC and a voltage regulator (like LM317) for the preamp tubes heating? Is this better than DC biasing and is it worth the effort?

6. Does star grounding really help?

7. Iím a bit cautious about using relays and/or photoresistors for switching between preamp channels because Iím afraid they may introduce noise into the signal path. So, rather than actually switching between channels, I plan to use a DPDT switch to ground the channel which is not being used. Each Master Level Pot output of each channel will go to a 0.022uF cap in series with a large resistor (e.g. 220k) and these resistors will connect to the input of the phase inverter. I will be grounding with DPDT the Master Level Pot output of the channel which is not used and will hope the sound of the other channel will not change much due to the relatively large resistors. Is this approach applicable? Any suggested resistor values! Any other ideas!

8. I may end up not needing one half of ECC83. What should I do with it - remove the heating wires and ground pins 1, 2, and 3, orÖ.? Will the other half of the tube be quieter compared to if both halfs were used?

Well, from the questions, most of you can probably tell that Iím a ďnewbieĒ, but please donít laugh.
Thanks!
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Old 20th April 2006, 12:18 PM   #2
Trout is offline Trout  United States
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Quote:
Iím not sure of the ampís model and I havenít completely traced the schematic yet. The logo on the chassis reads ďDinakordĒ which is quite of a misspelling of the European company Dynacord

There is actually a guitar amp built by Dinakord.

Would this be the amp you have?

http://www.atlantik.com.tr/atlantik/100s.html

I know nothing about that company, But it appears they handle a lot of sound equipment. Yours may be something they make or once made.

Quote:
2. Is there a limit to the value of PS filter caps? To reduce noise, I plan to use as large caps as practicable, e.g. two caps 470uF/450V - one before, the other after the choke, and five or six 220uF/450V for the driver and preamp stages, decoupled with resistors. Iím not sure whether using too large caps has any drawbacks..
Typically in a valve guitar and with tube rectifier you rarely if EVER see more than 30-40 uf on the first Cap. Generally old high power fenders used even less.

I have a 150W tube amp with SS rectifier that uses 2X50 (100) uf for the first cap position.

Power supplies are not the only source of possible hum. In fact, I have a Fender deluxe that has a total of 70UF filtering in the power supply and I get no adverse noise.

Trout
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Old 20th April 2006, 02:34 PM   #3
poobah is offline poobah  United States
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1) Don't know about the choke.

2) Yes, there is a limit on capacitance. To much and the current surge at turn on could harm your rectifiers... depends, Could be you really need NEW caps... not bigger ones... they do age.

3) Fine.

4) No, the winding prbably does not have the current rating.

5) Put in NEW caps first and see if that cures it. DC heaters are nice, but you should start with an auxiliary transformer.

6) YES.

7) Your input jack should automatically ground an unused channel.

8) Forget about it.

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Old 20th April 2006, 07:53 PM   #4
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Hi
Thanks for the replies will be of great help to me!

Quote:
There is actually a guitar amp built by Dinakord. Would this be the amp you have? http://www.atlantik.com.tr/atlantik/100s.html
No it isn't
I browsed the internet a bit and I think you're right, Dynacord probably never made tube amps for guitar, only a few models for vocals and general instrument applications which have been long ago discontinued. What I found is that my amp is an exact copy of the Fender Blackface Tremolux Head, and I meen exact to the very details like the holding plates on top, the knobs, the bright switches, and everything.....except for the logos. Even the placement of tubes and transformers on the chassis is the same. I will have to compare the schematics too. Strange, uh,...well ......maybe Fender sold in europe through another company (the amp has 220 to 110 V switch) or maybe my amp is just a well made fake.
Here's a pic of a Fender:
http://www.andrewsamplab.com/Amp%20P...20Tremolux.htm

Quote:
7) Your input jack should automatically ground an unused channel.
My question was a bit unclear. I wish to end up with some kind of an external pedal switch, so can switch b/n channels without unpluggin the guitar.

Thanks again. If I successfully finish this project I'll make sure to brag about it and post a picture and an MP3
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Old 20th April 2006, 08:49 PM   #5
poobah is offline poobah  United States
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I got it...photo resistors are a good choice...

Guitar amp aren't really that crucial when it comes to a little noise... I mean it is Rock and Roll.


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Old 21st April 2006, 09:29 AM   #6
SHiFTY is offline SHiFTY  New Zealand
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1) The output tubes are push-pull, so power supply noise gets cancelled out. The choke is there to filter the power supply for the preamp stages. Its a good setup, means you can use a lower current rated choke. Leave it as it is.

2) I would leave the power supply cap values as they are, tube heads have a cool sag thing that happens when you max it out, the voltage drops and you get crazy overdrive. Listen to Hendrix you can hear it happening.

3)No problem. Make sure they are away from heat sources.

4)Voltage divider off B+ definitely

5)Might be worth it- however if you only have a 6.3VAC winding you won't have enough voltage. Suggest trying other mods first.

6)Multiple star grounding works VERY well. One star for signal level grounds, one for power supply grounds, one for output grounds, bring them all together at one point. Run all low level signal in shielded cables. Cures nearly all hum problems, guaranteed

7)A relay should be ok, anything after the first stage doesn't suffer from induced noise too bad.
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Old 25th April 2006, 05:46 PM   #7
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Thanks Shifty and to everybody else again. I didn't think many would respond to a post so long . I hope I learn enough about the mystery of tubes and be able one day to help somebody myself. For now I'll go with the mods and hope I end up with a Rock'n Roll Monster Machine.
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