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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Sun Oct 16, 2011 7:09 pm
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What voltage do you have at pin five?

Arjay

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Mon Oct 17, 2011 1:44 am
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sorge wrote:

Is it possible that this socket is receiving no bias current and therefore sort of behaving like class B, with no cross over?


No bias current = tube red plating and you'll read on your bias probe a very very hot bias.

Theoricaly , in your case you should receive a very high bias voltage on grid like -50vdc if you read nothing with bias probe.

Fender Twin 65 RI are printed circuit or point to point ? Printed circuit ; someting is open / break somewhere in components path to this tube.

A careful looking and follow carefuly the circuit.


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Mon Oct 17, 2011 4:37 am
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Thanks again for all the suggestions.

Ok, checked again and R50 screen grid resistor (I think) is definitely burnt out (it's open circuit) I will replace this with another 1 watt 470 ohm as called for in manual. Are there any other requirements for this resistor?

I measured the remaining 3 and got a bit variance, 468 ohm, 475 and 462, within 10% so this is ok right?
Also I just put a new set of tubes in, is there any damage to the tube in socket V7? Was the tube just not running? Is there likely any damage to the amp from running it like this?

I took it to a tech I was going to with this issue (as he put the original tubes in that blew) and he told me there was nothing wrong with it which I found hard to believe. never going back there again.

Thanks very much for all the help!


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Mon Oct 17, 2011 7:31 am
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Good job sorge

The screen grid resistors should be the flame-proof type (Fp indicated on schematic). The other resistors values are fine.

Losing a screen grid resistor would not usually damage the tube or the amp, but a bad tube could cause the resistor to blow. If you ran the amp for long with one tube not working, there could be a mismatch of the tubes at worst. Just make sure that the current of this tube is close to the others and you're probably good.

It is possible that the tube socket has some damage from before, but a close visual inspection of the socket pins would reveal most problems such as a burnt pin. Use a magnifier to look closely. I don't think that a socket problem would cause the screen grid resistor to fail, but just about anything is possible.

I am wondering if the screen grid resistor was missed from the last time that your amp was repaired. :?:

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Mon Oct 17, 2011 8:30 am
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+1 to all of shimmilou's remarks.

I would, as a matter of course, replace all four of the screen resistors with fresh mil-spec types.

And it sounds like you've learned quite a bit from this ordeal -- the next time you have an amp problem, it's likely you won't need to call an amp tech because you'll be troubleshooting and repairing it yourself.

Rawk on!

8)

Arjay

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Mon Oct 17, 2011 7:24 pm
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I will replace all screen resistors when they come (hopefully today). Definitely learned alot and next time my amp has trouble I will have a go at fixing. Yes I think this particular screen resistor was missed, as I have run through 3 sets of power tubes with this issue ie. not been able to measure bias current on this socket. I thought I was going through tubes a bit fast, I do play pretty loud (with a drummer) and often and I guess running on 3 tubes doesn't help.

This forum is very helpfull thanks again for all the suggestions.

Just one last thing, slightly off topic, I am thinking about giving kt66's a go in the fender twin re issue. I realise the current draw for the heaters will be very close to what the transformer can supply. The tung sol KT66 has a draw of 0.9 mA per tube while all others I can find draw ~ 1.3 mA (I think).
Any recommendations/suggestions ie. go with tun sols (have to remove bear traps) or others should be ok with the others?


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Mon Oct 17, 2011 8:22 pm
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The 6L6 also draws about .9 A of heater current each, so sticking with a KT66 of the same draw would be a safer bet. Four 6L6 draw 3.6 A total heater current, and if you use a KT66 with 1.3 A draw each, that would be an additional 1.6 A of current for four tubes, or 5.2 A total heater current, which is a substantial increase from 3.6 A. In this case, I would want to be sure of the rating for the power transformer before using any KT66 with a heater current higher than .9 A.

Maybe contact Fender and see if the transformer can handle the increase in current draw when using the KT66 with higher heater current. My guess is that you would need a beefier power transformer, so as not to push the stock one too hard. But, it's your amp, and as Clint said; "You feel lucky?"

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Mon Oct 17, 2011 9:57 pm
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A suggestion only. If the 470-ohm screen resistor sits on the power tube socket (like between pins 6 and 4)--- you may see another resistor (the the grid stopper) sitting between pins 1 and 5. I usually replace BOTH with good flameproof, high temp, metal film resistors. Lengths of the resistors must be as short as possible, to help stability. It's why they solder them onto the socket tangs. Just an FYI...

http://www.thevintagesound.com/ffg/sche ... layout.gif

Takman resistors are a good choice (IMHO) for these critical positions. Helps to keep the amp stable.

http://www.partsconnexion.com/resistors ... _watt.html

Data Sheet:

http://www.partsconnexion.com/prod_pdf/takman_rey.pdf


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 18, 2011 12:18 am
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Great, thanks for the info shimmilou, I think I will go with the tun sols as I am not feeling lucky and not prepared to risk it with gigs etc.

Thanks for the info BMW2002Ti. I don't really know about this but from reading a few articles on the net I have read that carbon composite/film give a more vintage sound (could be bullshit but article below says in high voltage swing situations there is some truth) anyway I can source metal oxide film resistors at work, but not carbon composite. Is it a good idea to stick with carbon composite in both the grid stopper and screen resistors or are metal oxide film ok in these applications?
I read the link you sent and see that those paricular metal film resistors are specially designed to be comparable to carbon film etc.

http://www.geofex.com/article_folders/c ... oncomp.htm


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 18, 2011 12:54 am
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Sorge, Allen-Bradley CC's are fine in about 95% of the amp. But, these metal film resistors are better (in the long run) in areas that are hot (like under the power tubes). They tend to drift much less under those conditions, than CC's. And I feel their effect on the tone is not detrimental.

But, their ability to keep a tight voltage control under extreme conditions will keep your power tubes much happier, IMHO. It's critical that the screen voltage stay at or below the anode plate, under heavy transient loads. And the grid stoppers are there to filter out ultra-high frequency oscillations and other instabilities.

For me, CC resistors tend to drift a bit too much sitting under the power tubes. And that can have as much effect on tone and stability, as the material the resistor is constructed with. I only use the Takman in these positions. And low-inductance, wire-wound for cathode biasing single-ended amp power tubes. And also in the power supply chain. Most gain stage, etc get or keep CC resistors.


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 18, 2011 1:14 am
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BMW2002Ti wrote:
Sorge, Allen-Bradley CC's are fine in about 95% of the amp. But, these metal film resistors are better (in the long run) in areas that are hot (like under the power tubes). They tend to drift much less under those conditions, than CC's. And I feel their effect on the tone is not detrimental......For me, CC resistors tend to drift a bit too much sitting under the power tubes. And that can have as much effect on tone and stability, as the material the resistor is constructed with......Most gain stage, etc get or keep CC resistors.


+1

I've got A-B carbon-comp screen resistors in most of my amps -- primarily to preserve the visual vibe of the chassis. Generally they're stable for about ten years, about the same as a set of filter and bias caps in an amp that regularly gets pushed hard. I installed metal-film screen and grid stopper resistors in my ultra-linear Twin Reverb since it's a thermal brute, with plate voltages in excess of 500vdc and power tubes as hot to the touch as EL84's. The Takman's are both durable and stable -- easily good for 12 to 15 years before any drift is likely to occur.

Arjay

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 18, 2011 1:56 am
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+1 Retroverbial ,,
I go too for carbon composite in all my vintage amp.

Or cement 5 watts 470 ohms screen resistors


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 18, 2011 8:07 am
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Ok cool, interesting to hear different peoples take on the resistor types and how important they are in certain positions.
I replaced the blown screen resistor with a metal oxide flameproof, and now its all working sweet. Get plate voltage and bias current on the tube socket and also see the tube pulse blue when I play, like the others.

Will replace the remaining ones on the weekend when I have a bit more time.

Cheers once again for all the help.

Now to separate the combo, eminance delta pro 12a's just make it too damn heavy................ and also get a bassman :) :)


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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 18, 2011 5:55 pm
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How's it sound now, Chris?

Regards,
Roger

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Post subject: Re: Fender Twin 65 RI - bias and tube question
Posted: Tue Oct 18, 2011 11:32 pm
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Sorge, just an example of what I try to do. To keep the unit and the tubes happy. Yet, keep the vintage tone.

Old Reverb Unit (OEM parts):

Image


Redone with low inductance resistor on 6K6GT cathode, new bypass caps (rated at 50VDC), all else was within specs (10%) and remained in unit:

Image


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