Fender Dlx Rev build- last Q b4 fire-up.

[eschertron]

ChuckH- would a 1/2W 10r do? my store dont do 1W r's.. and I got plenty of 1/2w but smallest 10r (s'one said they use 10r for this addition anyway).

Thanks, Sea Chief

I'm not Chuck, but I stayed at a Holiday Inn Express last night (Oh, wait... do they have that commercial in GB?)

The Rs you mention won't do as bias sense resistors for a couple reasons:
1) 10 Ohms will burn 100 times the power of a 1R resistor (given a constant current), so they'd need to have a much bigger wattage rating (rather than smaller) to have the same safety margin.
2) While a 1R in series with the cathode will have an insignificant effect on bias voltage - in the range of mv - and is generally ignored in the resulting bias calculation, a 10R resistor could throw the bias calculation out of whack if the voltage drop across the resistor wasn't taken into account. Also you'd have a kind of hybrid cathode-bias that might, to some golden ears, be perceptibly different from the design.

M2CW - wait til you get the 1R resistors, or simply forget about it and get a bias probe.

[Chuck H]

What eschertron said.

A 10R could work fine if you used an appropriate wattage rating. More on the order of 2W for safety margin (3W more commonly available). More resistance = more voltage drop across a greater resistance = more watts. With a 1R/1% the bias calculation is 1mW = 1mA, simple Simon. With a 10R you need to change the math. Not a huge deal breaker and I've seen some builders do it like this. It's all multiples of 10 so the math isn't the issue. The need for a higher wattage resistor might be (since you don't have them) or the "golden ears" effect that eschertron mentioned. A 10R would have some self regulating affect on the bias. Making your amp a "combination bias". Not a bad thing unless you subscribe to internet lore about some Fender amps biased like this being inferior. I doubt 10R of cathode bias would make a significant audible difference, but some would surely disagree. Just go with the 1R/1W/1% resistors as suggested. It's not a race and the amp isn't going anywhere.

[Sea Chief]

Understood most of that escherton & ChuckH- thanks.

Ok Simple Simon it is- ordered 1r 1W from ebay. so still another few days and I'll be up & running with these soldered in place.

In meantime, with the new 5U4GB Im at 452v on the plates. It does sound a bit flat Id say with the 5U4GB in (whether it makes any sonic difference to the 5AR4 I don't know. And some other niggles..

1) the tremolo intensity: it seems as strong at my 10 as clips of std DRRI at only 3, its actually inaudible up to 5.

2) Reverb hum- vol increace = hum increace (reverb rca lead plugs new, reverb pan new). Reverb sounds good tho: 4 is ideal, 10 is ott wet IE as a DRRI is normally so I believe.

Cheers, SC

[Chuck H]

Sounds like the reverb is working correctly I never met a Fender reverb that didn't hum a little. And the ability to get an over the top wet mix is another signature. If the noise is genuinely hum at 60Hz you may hope to reduce it by altering the grounding scheme in that circuit. If it's 120Hz it could be EMF coupling from the PT to the pan transducers or circuit proximity of the reverb transformer to the preamp. The first thing to try is to unscrew the bag and move the pan around in the amp to find the quietest spot before screwing it back down. A difference of a half inch in any direction can make a huge improvement sometimes. You can also try a different driver tube, some are noisier than others in this way. Same type, just a different tube. Lastly, unbolting the reverb transformer and moving/rotating it around to find a quiet spot can help sometimes. If what you have is actually buzz (not 60Hz or 120Hz, usually above) this could also be a noisy tube, driver or recovery, or it's just a byproduct of gain. Reverb circuits are actually very high gain because they must amplify a tiny signal from the pan and a high noise floor is the result. You can try a shielded cable from the pan to the recovery tube. IMHE if you polish all these aspects of the circuit it's very possible to get much quieter performance.

Are you using the reissue or BF type trem circuit? The voltages are important for a tube oscillator and to make the trem bug flash properly and your amp has about 60 more volts on the HV rail than the reissue and perhaps 35 more than an original. Some of that very high voltage may be due to a cold bias condition that we haven't checked for yet. I wouldn't sweat the trem until you get the amp biased proper so the HV is known. Then you can check voltages on the trem circuit and possibly make adjustments.

[Sea Chief]

Hi again ChuckH.

The reverb.. actually Ive just noticed last 2 days a bad feedback sound with the reverb. It sounds like a typical gtr one, but unable to stop it with a gtr, so its the reverb doing it. I have to turn the dial right down to stop it as t increaces and increaces to really bad. very odd.

The trem circuit: its using the DRRI board, so the circuit is that.. but for the 'roach' which I could only find a Mojotone one (suited for Dlx Rev I did check). So essentially the RI circuit with a Mojo optoisolator (and of course the general HV voltages are 60+v higher than the 396v B+ on the schematic.. Im rather alarmed about, esp with a 5U4GB in). Trem seems simply not 'strong enough', and a bit of 'motorboating'(is it?) ticking but not bad.

[Tom Phillips]

Sea Chief,
Some of the things you have been describing can be caused by wiring dress so it would be helpful for us to see photos of your build. Are you able to post photos yet?
Tom

[Sea Chief]

I tried moving the reverb right down into the EXt cab (the DR is in a head, same dims as orig fender head) but still hum and bad feedback.

thx SC

[Sea Chief]

Sea Chief,
Some of the things you have been describing can be caused by wiring dress so it would be helpful for us to see photos of your build. Are you able to post photos yet?
Tom

Good point Tom. I will get some photos of inside (& outside too).. when Ive time. Im working flat out at the mo sat & sun.. Im all behind as I made a cabinet for this just-finished amp! i think into photobucket > then onto here I think i did last time.

[Tom Phillips]

Posting photos to MEF

... i think into photobucket > then onto here I think i did last time.

I'm not sure what you mean by that but I think it's best to post the photos directly in this forum. (Reference post #70)

Even though there have been attachments lost here once there are far more links to offline photo servers that have gone dead.

Cheers,
Tom

[Chuck H]

I tried moving the reverb right down into the EXt cab (the DR is in a head, same dims as orig fender head) but still hum and bad feedback.

thx SC

60Hz hum is pretty obvious (to me and most I think). It sounds like a low bass note. 120Hz hints at sounding like what some call a buzz. An actual buzz sounds more complex like a "difference tone" and usually seems a little higher pitched. What sort of noise do you have?

It can be tricky to impossible to locate a reverb pan in a head cabinet without noise. I would expect the feedback to be much worse in the extension cabinet than in the head cabinet. Because it's acoustic feedback, the less isolated the pan is from the speakers, the more it will feedback! Try separating the head and cabinet by a few feet and see if the feedback problem continues. If it does check to see that the tube isn't ringing microphonic. Disconnect the pan output and short the tip and sleeve. Now turn up the reverb. Does it start to ring? Tap the recovery tube to see if it's super sensitive. If it's positively the pan then you can try additional acoustic damping around the pan inside the bag. But it sounds like you have a real bad case and it can be very frustrating. I've read that a lot of builders are having better luck with the three spring short pans than the old school two spring long pans. I have customer with a reverb that is just barely stable. Right now it's wrapped up like a sausage between layers of foam and bubble plastic and that barely keeps it behaving at setting 5. It's on my "I'll get to it" list because the customer never turns it above 3 but I plan to replace with a short three spring pan.

As has been mentioned, the extra HV could be partly due to a lack of current from a cold bias condition. Once the bias is set and the HV is known you can check voltages in the trem circuit.

[Sea Chief]

Right- finally some 1r resistors arrived, & apparantly 1W (as on the ebay listing).

They're metal film (& pink fwiw)... but seem to be only 1/2W physical size/ IE 10mm long x 3.2mm wide. Ive asked seller but no reply. So Im stuck firm/ I will not solder in until I know they are 1W and not 1/2W.

Any ideas?

[Chuck H]

If you know the make/brand you can probably look up a spec sheet on line to fact check the physical size. Some resistors can seem quite small. Part of a resistors rating has to do with heat dissipation and some of the smaller ones are simply made from more heat stable materials or in more sinister cases spec'd to be used/installed only certain ways. Like with a heat sink, under limited ambient conditions, in a mechanically vented circumstance or with clearance on all sides. I have a hot mess of MF 1W resistors that I've never used only because they're about the size I'm use to seeing for 1/4W resistors! They just "look" funny and I don't like 'em.

[eschertron]

Right- finally some 1r resistors arrived, & apparantly 1W (as on the ebay listing).

They're metal film (& pink fwiw)... but seem to be only 1/2W physical size/ IE 10mm long x 3.2mm wide. Ive asked seller but no reply. So Im stuck firm/ I will not solder in until I know they are 1W and not 1/2W.

Any ideas?

+1 to tracking down manufacturing data. However...



The 1W 1R resistors I got look like little brown "Mike n Ike" candy - rounded and smooth, not like most MF (or CC) resistors that I've seen. They are also about the size that I'd expect from a 1/2W resistor. From a reputable source, though (Mouser) so I trust them.

I've seen resistors that look like that before. Just odd-looking, that's all.

[Sea Chief]

If you know the make/brand you can probably look up a spec sheet on line to fact check the physical size. Some resistors can seem quite small. Part of a resistors rating has to do with heat dissipation and some of the smaller ones are simply made from more heat stable materials or in more sinister cases spec'd to be used/installed only certain ways. Like with a heat sink, under limited ambient conditions, in a mechanically vented circumstance or with clearance on all sides. I have a hot mess of MF 1W resistors that I've never used only because they're about the size I'm use to seeing for 1/4W resistors! They just "look" funny and I don't like 'em.

Still cant get any answer from seller. Until I know defo what they are apart from just "Metal Film 1r/ 1Ohm, 1W".. Im not gonna bung em in. They look identical size to all those carbon comps in a p-to-p Champ I have. Frying my tubes by a 10p 1/2W resisitor (if they are) is lunacy.

Understand all that ChuckH- good info there. I hope as theyre Metal Film then yes, they can be smaller than usual.. but as I often say, assumptions + amps = arsebanditry.

[Chuck H]

FWIW there is absolutely no danger to the amp or tubes by plunking them in there. The worst that would happen is that one or both fail open. In that case the tube/s simply won't conduct. More a problem for you than the amp