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Post subject: More Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 9:57 am
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I'm sorry to start a new post, although it's about this PR I just received https://forums.fender.com/viewtopic.php?f=12&t=110696 I figured it's not NAD anymore. Now I have some questions for you knowledgeable, experienced, fine forum members.
1st question) For those two 18K 1 watt CC resistors, one on the power-rail and one on the filter cap, should I use 18K 2 Watt Metal Oxide instead or ......?
2nd question) The bias cap, should I use 50uf at 100 vdc or 100uf at 100vdc, and does it matter, I have both. My research tells me it is debated which to use, not trying to start a debate, just want to know
Thanks in advance, most of my parts arrived, hope I get some time this weekend
mud


Last edited by mud on Tue Jan 08, 2019 1:35 pm, edited 2 times in total.

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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 10:18 am
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I did not read your first post,
For your question now ;

More watt is better than less , use 5 watts power for rail resistor. They are standard and may cost less than a 2 watts.
IMO your original resistors are 2 watts not 1 watt

https://www.tubesandmore.com/products/r ... ound-power


Caps are in Microfarad not ohms wich are for resistor
For bias, 100 mfd caps are better ( more quiet ) than 50 mfd, use one at 100 volts DC

Check fuse if it is the right one. A too powerful fuse won't protect your amp and could blow your power transformer


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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 10:35 am
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stratele52 wrote:
I did not read your first post,
For your question now ;

More watt is better than less , use 5 watts power for rail resistor. They are standard and may cost less than a 2 watts.
IMO your original resistors are 2 watts not 1 watt
All of the schematics I saw say 18K 1 watt

https://www.tubesandmore.com/products/r ... ound-power


Caps are in Microfarad not ohms wich are for resistor
For bias, 100 mfd caps are better ( more quiet ) than 50 mfd, use one at 100 volts DC

Check fuse if it is the right one. A too powerful fuse won't protect your amp and could blow your power transformer

:oops: It's early yeah mfd not ohms I do know that, but I had resistors on my mind. It did have a 3amp quick blow I did change it for a 1 amp slow blow
mud


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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 10:52 am
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mud wrote:
. It did have a 3amp quick blow I did change it for a 1 amp slow blow
mud



Yes, yes and yes.
Do not power amp until you put the right fuse. Your amp is not protected :shock:


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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 10:56 am
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Metal oxide resistors are fine for the power rail and bias supply but I would not increase the wattage rating. There is nobody here on this forum who knows as much as Leo did and everything he did, he did for a reason.

The 100/100 bias-supply cap will work fine and usually helps to tighten up the amp's overall bass response, ie: less audible flabbiness. However if the speaker is not up to the task, the improved bass may actually be counterproductive.

Arjay

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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 12:21 pm
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Retroverbial wrote:
Metal oxide resistors are fine for the power rail and bias supply but I would not increase the wattage rating. There is nobody here on this forum who knows as much as Leo did and everything he did, he did for a reason.

y


Why put a 100 mfd bias cap if Leo put 50 ?

I'm sure you know more wattage or more volts are more expensive, amp builder ( leo or any amp manufacturer) look to put the cheaper parts in their amps.
For me , buying a 2 watts resistors at my electronic surplus store are more expensive than a 5 watts and it will last all the life.

We are lucky in the Leo's area there is only one quality in parts ; the best.


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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 12:50 pm
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Retroverbial wrote:
Metal oxide resistors are fine for the power rail and bias supply but I would not increase the wattage rating. There is nobody here on this forum who knows as much as Leo did and everything he did, he did for a reason.

The 100/100 bias-supply cap will work fine and usually helps to tighten up the amp's overall bass response, ie: less audible flabbiness. However if the speaker is not up to the task, the improved bass may actually be counterproductive.

Arjay

Thanks being I have both I may try one and then try the other and see what I like better.
mud


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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 1:03 pm
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stratele52 wrote:
Retroverbial wrote:
Metal oxide resistors are fine for the power rail and bias supply but I would not increase the wattage rating. There is nobody here on this forum who knows as much as Leo did and everything he did, he did for a reason.

y


Why put a 100 mfd bias cap if Leo put 50 ?
It seems like it has to do with the bass response, and as Ajay says, not his quote, but the speaker being used will dictate whether the 50mdf or the 100mdf sounds better. I read things like the 100mdf will tighten up the bass, if that is what one is looking for. Being either value won't hurt anything I will give both a try, and and than stick with the one that sound best to me

I'm sure you know more wattage or more volts are more expensive, amp builder ( leo or any amp manufacturer) look to put the cheaper parts in their amps.
For me , buying a 2 watts resistors at my electronic surplus store are more expensive than a 5 watts and it will last all the life.

We are lucky in the Leo's area there is only one quality in parts ; the best.

Thanks
mud


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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 1:05 pm
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The human ear being *somewhat* subjective, you may or may not notice an audible change between the two different bias-supply caps. But regardless, the higher value should provide a cleaner negative DC voltage (be sure you polarize it correctly).

Arjay

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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 4:54 pm
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As far as resistors wattage goes, just check the spec sheet and ensure it covers what amperage/voltage your amp will see. Probably best to go w/ what meets the criteria of the amp, the 1 watt metal oxide may be sufficient.

The "dielectric" max voltage ratings for AB CC 1 and 2 watt resistors were 1000v & 1500v respectively unless you were operating at significant elevation 50k feet = 625 volts. While 1/2 watt could handle 700v/450v at sea level and 50k feet.

AB CC resistor info:
Wattage rating body Dimension(in inches) RMS Voltage
1/8 .145 150
1/4 .25 250
1/2 .375 350
1 .562 500
2 .688 750

My take on the capacitance adding to bass response, it takes more power to reproduce lower frequency accurately. More storage capacitance does this and reduces time to recharge which allows better tube operation, within the ratings of the tube. Review the tube data(rectifier) sheet for max capacitance and stay w/in about 10%.
For bias, the spec sheets I have access to AA1164 show 25uf x 50vdc w/ -32vdc bias. I noticed in one of your photos you have the AB763 layout are you gonna blackface this amp? If so, higher voltages should be anticipated w/ a GZ34. Either of these imo, will do the job 25ux50v, 25ux100v,50ux100v,100ux100v as long as bias is maintained.


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Post subject: Re: Questions and Help with 68 PR
Posted: Fri Sep 21, 2018 9:48 pm
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Bass response has a lot to do with output trannys. https://forums.fender.com/viewtopic.php?f=12&t=100159
What bias cap is in it now? If 25 uF, I would change it to 50uF. Since bias is fixed in this amp, it should be checked.
For the resistors I would go with 2 Watt, metal oxide. The only reason for Carbon comp is this, IMO.
http://www.geofex.com/tubeampfaq/taffram.htm

Sorry, part way down the page is an explanation by the author of carbon comp resistor "mojo"

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Post subject: Re: Questions and Help with 68 PR
Posted: Sun Sep 23, 2018 1:29 am
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I've been rebuilding power supplies using metal oxide and wire wound resistors, for about 20 years now. They've been very reliable. I use 2 and 3-watt metal oxide. 5 and 12-watt wire wound (primarily MIlls) resistors.

https://tshop.r10s.com/a08/a2b/8855/071 ... 7376dc.jpg

http://www.soniccraft.com/images/Vishay_Mills-960.jpg


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Post subject: Re: Questions and Help with 68 PR
Posted: Sun Sep 23, 2018 9:46 am
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Looking at the spec sheets(tubes&more/AES), 5 watt metal oxide is the minimum w/ 400ish volts to the plates. The 1/2/3 watt metal oxide would be stressed after looking at its spec sheet(working voltage=350), while it may work it would drift over a period of time. The interesting thing looking at the spec sheets, carbon film resistors offer greater working voltages than the metal oxides in the same wattages. While MO offer a higher heat rating. Wire-wound are certainly an option as well.

I would suggest shopping from a dedicated electronics distributor for better choices. As there are certainly more manufacturers/options than what are available from most guitar/amp sites for your needs.

sorry for all the edits.


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Post subject: Re: Questions and Help with 68 PR
Posted: Mon Sep 24, 2018 1:06 pm
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sfceric64 wrote:
For bias, the spec sheets I have access to AA1164 show 25uf x 50vdc w/ -32vdc bias. I noticed in one of your photos you have the AB763 layout are you gonna blackface this amp? If so, higher voltages should be anticipated w/ a GZ34. Either of these imo, will do the job 25ux50v, 25ux100v,50ux100v,100ux100v as long as bias is maintained.

It is in my belief that this is a AA764 Blackface circuit, it came with a GZ34, built in 20th week of 1968, and the tube chart does say AA764 ( I know that you can't trust the chart), but I do believe this is what it is, which is a blackface version of the Princeton Reverb
This is quite a mystery
mud


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Post subject: Re: Questions and Help with 68 PR
Posted: Mon Sep 24, 2018 2:11 pm
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Well I had some time this Saturday, and some parts so I replaced all the electrolytic caps
ImageI went with the 100uf 100vdc bias capImage
Grounded plugImage
Image
After this I resisted turning it on, as all I own are older tubes, and in case there is a real issue I did not want to pop any, so I ordered two EH 6v6gt at $14 each I already own a ruby 5AR4 I pulled the Mullard GZ34, and orderd 18K resistors for the power rail and the filter cap, I measured the resistor while I was in there and both are within 10% tolerance but just barely. Did a big cleaning of the knobs face tolex, now just waiting on tubes
mud


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