Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fkadi on February 11, 2025, 07:52:06 am

Title: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: fkadi on February 11, 2025, 07:52:06 am: I have a 1979 Fender Twin Reverb Ultralinear 135 watts that is in working order. I bought it around a year ago with no issues. However recently I took out the chassis and did a visual check and found that it is missing the 30K 20W resistor. I have included a picture of my chassis and a schematic with the resistor I am referring to.

I believe this is a voltage dropping resistor working with the 2700 ohm resister to achieve the +410V, with it missing this would logically mean the voltage is higher than expected. I am unsure who or why this was removed and my question is why would the resistor be removed, could there be a technical reason? and what is the effect of it being removed? would this cause damage to the circuit? etc...

Again the amp is working and has been used off and on for a year with no issues.

(Note: the amp in a 120V version with an external step up transformer for use with 220V mains.)

Thank you
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: pdf64 on February 11, 2025, 08:14:41 am: As you note, the 2k7 and 30k resistors form a potential divider to reduce the HT voltage at nodes other than the OT CT.
And yes, without the 30k, the HT node voltages will rise, which may subject the HT caps to excessive voltage eg at power up, and will increase the V3 reverb driver dissipation and voltage stress.
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: Latole on February 11, 2025, 02:42:47 pm: pdf64 is right
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: fkadi on February 12, 2025, 12:05:48 am: Quote from: pdf64 on February 11, 2025, 08:14:41 am
As you note, the 2k7 and 30k resistors form a potential divider to reduce the HT voltage at nodes other than the OT CT.
And yes, without the 30k, the HT node voltages will rise, which may subject the HT caps to excessive voltage eg at power up, and will increase the V3 reverb driver dissipation and voltage stress.

Thank you for your reply, yes exactly that may cause damage later on. I will need to replace that resistor for sure. I have been looking online for one and I can't seem to find one with the same specs. Do you think it can be replaced with different specs? lower wattage? place a couple in series?

Thanks again for your help and feedback.
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: SEL49 on February 12, 2025, 12:23:04 am: eBay 30K 20W resistor (https://www.ebay.com/itm/204188848520?_skw=30K+20+watt+power+resistor&itmmeta=01JKWCSHSYCX5S82MCTDC1J575&hash=item2f8a9a9188:g:VFYAAOSweP1jnSmq&itmprp=enc%3AAQAKAAABAFkggFvd1GGDu0w3yXCmi1fl43GIMw1zq4p6vF4ke%2BCc8b4jUStWUak%2F6Pqn0Ka9Yi9HBdRqOUmrYKH2FTECYT60a0PPp0d4ENhht9yOST6XTcZZwEWOrSBdzDWihWGPftaOOgcRoQRMJDT1V8Qc9FYTu9i2%2BEhm1DgIY3GUP19nZFgIPGY36Dduoa%2Fexbt4yBfxOwqepXNdp8hHf4NZ6wZX9y6r0Sj4nzHbkeXMGRKS0FNFBNFEYW%2F6SmQUNv4ayhVKOgn6wG8O8KEJeHA8APw9PeR7KDg3Q1gtD6I%2FMOFUvZLw0TwCQ26ipWjnAFRvBUgf5g6ZggoogxnNp58Tbgk%3D%7Ctkp%3ABk9SR6Sd5oyfZQ)
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: fkadi on February 12, 2025, 12:42:31 am: Quote from: SEL49 on February 12, 2025, 12:23:04 am
eBay 30K 20W resistor (https://www.ebay.com/itm/204188848520?_skw=30K+20+watt+power+resistor&itmmeta=01JKWCSHSYCX5S82MCTDC1J575&hash=item2f8a9a9188:g:VFYAAOSweP1jnSmq&itmprp=enc%3AAQAKAAABAFkggFvd1GGDu0w3yXCmi1fl43GIMw1zq4p6vF4ke%2BCc8b4jUStWUak%2F6Pqn0Ka9Yi9HBdRqOUmrYKH2FTECYT60a0PPp0d4ENhht9yOST6XTcZZwEWOrSBdzDWihWGPftaOOgcRoQRMJDT1V8Qc9FYTu9i2%2BEhm1DgIY3GUP19nZFgIPGY36Dduoa%2Fexbt4yBfxOwqepXNdp8hHf4NZ6wZX9y6r0Sj4nzHbkeXMGRKS0FNFBNFEYW%2F6SmQUNv4ayhVKOgn6wG8O8KEJeHA8APw9PeR7KDg3Q1gtD6I%2FMOFUvZLw0TwCQ26ipWjnAFRvBUgf5g6ZggoogxnNp58Tbgk%3D%7Ctkp%3ABk9SR6Sd5oyfZQ)

Thank you. This is a different kind but I presume it can be used as a replacement for the cement/ceramic type that was probably originally in the amp?

Thanks
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: fkadi on February 12, 2025, 06:22:07 am: From your experience would a 33K 10W work as a replacement since I was able to find one available?

Note: I am not in the US.

Thank you
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: SEL49 on February 12, 2025, 07:26:36 am: According to the schematic there will be 410V across the 30K. P=E²/R= 410²/30000=5.6W. This is the actual power dissipated by that resistor. Now double that for a safety margin and you need a power rating of 11.2 watts. If I had a 30K 10W in hand, I'd use it in my own personal amp. If repairing for someone else's amp I'd buy a 20W resistor.

eBay is a great source for electronic components. Search eBay in your area.

BTW, that Clarostat resistor on eBay is top quality.
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: mresistor on February 12, 2025, 07:48:53 am: Can you order from Amazon in EU? https://www.amazon.com/Wirewound-Ceramic-Horizontal-Cement-Resistor/dp/B0D9N4Z68K/
Title: Re: Fender Twin Reverb Ultralinear 135 watts 1979 - 30K 20W Resistor missing !!
Post by: fkadi on February 13, 2025, 12:39:01 am: Thank you all for your help, I am still baffled as to why anyone would remove this resistor, but I will definitely replace it as soon as possible.
Thanks again