Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on April 26, 2021, 02:54:24 pm
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At least that's the owner's complaint. He is a pro and gigs a lot. This is his little gig amp meaning he runs it pretty loud. Based on his description-"It loses it's tightness and kind of sounds mushy after a while", I'm thinking it just needs tubes. On my bench at low volume it sounds fine. I don't hear any hum and the sticker date is 2019 so I don't think it needs caps...?
What say you?
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My experience is that kinda thing can be due to the speaker heating up, resulting in the voicecoil resistance rising, and the transducer becomes more lossy.
The problem is halved if an extension cab is used.
If not an option, a speaker with a beefier, vented magnet should help.
But of course, the tone will change.
An EVM12L will probably make issue history.
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he runs it pretty loud. Based on his description-"It loses it's tightness and kind of sounds mushy after a while"
Another option is simply ear fatigue from loud volumes which can cause things to sound mushy.
With respect, Tubenit
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he runs it pretty loud. Based on his description-"It loses it's tightness and kind of sounds mushy after a while"
Another option is simply ear fatigue from loud volumes which can cause things to sound mushy.
With respect, Tubenit
This was my first thought but I immediately had to dismiss it because he plays other amps (AC30, 2203) that he doesn't have this complaint with. His BD does have a Celestion G12t-75 which I've always thought of as being a pretty robust speaker. Although the BDRI is 40 watts, he pushes it pretty hard with drive pedals. Regardless, he's getting new tubes and we'll take it from there.
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At least that's the owner's complaint. He is a pro and gigs a lot. This is his little gig amp meaning he runs it pretty loud. Based on his description-"It loses it's tightness and kind of sounds mushy after a while", ...
What say you?
The BD is well known for a variety of problems -many of which are associated (one way or another) with the overheating of 5W cement resistors on the main board. So that’s a good place to start...
https://m.youtube.com/watch?v=3259fb4w37I (https://m.youtube.com/watch?v=3259fb4w37I)
https://m.youtube.com/watch?v=zaLN5MSnPTg (https://m.youtube.com/watch?v=zaLN5MSnPTg)
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An explanation ; a new toy is great the first weeks. After few years it came ordinary.
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At least that's the owner's complaint. He is a pro and gigs a lot. This is his little gig amp meaning he runs it pretty loud. Based on his description-"It loses it's tightness and kind of sounds mushy after a while", ...
What say you?
The BD is well known for a variety of problems -many of which are associated (one way or another) with the overheating of 5W cement resistors on the main board. So that’s a good place to start...
https://m.youtube.com/watch?v=3259fb4w37I (https://m.youtube.com/watch?v=3259fb4w37I)
https://m.youtube.com/watch?v=zaLN5MSnPTg (https://m.youtube.com/watch?v=zaLN5MSnPTg)
Nice info, thanks for posting those videos, I had a similar issue with a Peavey Classic 50. The five watt cement resistor was totally burnt and open. Replaced with two five watts in series for 10 watts, fixed the issue. elevated them off the board.
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In looking for these 5w cement resistors, I found that this amp doesn't match any schematic I can find. The resistors are labelled R78 & R79 and are 470Ω. Schematics don't have R78 or R79. Don't know how critical it is, but I'd like to have a correct schematic. The main board has 2017 silkscreened on it if that helps.
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This schematic (p2) has those resistors as 470ohms https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Deluxe-Reissue-2004-Schematic-Rev-A.pdf
But if in doubt, ask Fender for the correct tech info, quote the serial number etc.
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Well I just checked the bias with the original tubes in and it's reading 35mv. Since the test point is tied to both tubes (visual confirmation), and everything I've read says it should be in the 60mv range I'm thinking this has been retubed (sovtek) and biased incorrectly. I'm surprised it sounds good at all with the bias that low, but what do I know? What do you pros say? Can it be this far off and still sound reasonable?
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Yes.
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Different tubes, different circuit, so may not apply, but I suspect it does . . .
I got tired of my Blues Juniors EL-84 tubes burning up frequently, and wondered if re-setting bias to lower plate dissipation would help me achieve longer tube life (spoiler alert, it did not make much difference in these amps). So I biased down to 40% max plate dissipation before I heard any difference in tone, left it at 50% and it played great, insofar as a BJ plays great.
Our tube amps are funny beasts -- very tolerant of many, but not all, abuses.
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Is it normal for the bias to change while playing thru the amp? I happened to have the meter still attached while playing thru the amp and when i play a big chord the bias shoots up to 100mv. When I stop playing it settles back to 65mv.
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That's normal for fixed bias output tubes.
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What you’re measuring there is signal conditions.
Bias is checked at idle = no signal.
With AB amps, anode / cathode current in the output valves increases at higher signal levels.
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So here we are again. I retubed it back in May and he called me yesterday and asked if I could look at it again. Same issue-after 2 or 3 sets, the amp just starts to sound flat or thin. Sounds to me like it's losing it's dynamics. Tubes check out and the bias was at 72 mv. I brought it down to Fender's recommended 60mv, not that it matters.
What could cause an amp to lose it's...Mojo? I can guess that heat is the culprit, but what components? I notice that the screen resistors get really hot really fast, but I see no signs of the board getting burnt under them. They measure within tolerance of 470Ω. Any Ideas?
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I retubed it back in May
If he is really playing it hard then it wouldn't be unreasonable to wear out tubes again in that time period.When my amp loses mojo I replace the KT88 and instant mojo. About once a year when I was rehearsing once a week. I push hard.
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...I notice that the screen resistors get really hot really fast...
They shouldn't. Screen current on 6L6 runs 5%-10% of cathode current. Setup like this, cathode current will rise from 60/72mA at idle to 180mA-220mA at full roar.
Idle: 10% of 72mA is 7mA (0.007A). 0.007A in 470 Ohms is 3.5V, 0.025 Watts
LOUD: 10% of 220mA is 22mA 0.022A). 0.022A in 470 Ohms is 10.4V, 0.23 Watts
Screen current will go high if load is open-circuited or very-high. Is he maybe running an attenuator? Did I miss reading that?
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No attenuator. He just told me he runs it on about 2 with pedals pushing the front end.
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Different amp, but this vid is interesting https://www.youtube.com/watch?v=Q-22fKYratY (https://www.youtube.com/watch?v=Q-22fKYratY)
Also a different amp, but a friend has a Deluxe Reverb reissue. It would get hot after playing for a while and the sound would go off. It had developed tiny cracks in a few PCB traces. They would apparently open some when the amp got hot. Jumpering them and replacing the connected resistors solved that problem - at least for a while.
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One mistake I made was misidentifying the screen resistors. I've since discovered that this amp is a Rev. C and things are slightly different. The 470Ω resistors I ID'd as the screen resistors are actually part of the +- 15v PSU. But the actual screen resistors check out good and have no signs of heat.
One other thing I noticed is that when I bring the amp out of standby, there are some slight crackling noises thru the speaker for about 10-15 seconds. Then it goes away.
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> part of the +- 15v PSU.
When the solder joints on those low-volt rail dropper resistors go bad it can make all sorts of troubles.
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As a relative noob here...
If he's really beating up those tubes, maybe adding a 120v fan to take some strain off them? :dontknow:
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It turns out he isn't pushing this amp that hard. He tours nationally with a couple of different acts and in one of them I know he uses AC-30's and Naylor amps. This is his local cover band amp and he tells me he runs it using the clean channel only and the volume is at 2-3.
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It turns out he isn't pushing this amp that hard. He tours nationally with a couple of different acts and in one of them I know he uses AC-30's and Naylor amps. This is his local cover band amp and he tells me he runs it using the clean channel only and the volume is at 2-3.
Time to build him a reliable amp.
A single clean channel that will work every night is not difficult or expensive.
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I've been thinking of just that. Especially since he said if we can't fix it, he'll probably just sell it.