DC-5 Squealing Problem

[twohailmarys]

I bought a dc-5 combo yesterday and didn't have much time to try it out, but it was a really good price. the problem i am having is on the lead channel, when i turn the gain past 6 it makes a squealing noise. it even does it when the volume is barely on. i even tried turning all of the other knobs all the way off. it doesn't change when i turn off the volume on my guitar, or unplug it. I tried changing the preamp and power tubes with 2 different working sets and it still does it. is there anything else i can try?

 

[frets70]

Had the same problem with a DC-3 I bought recently.
It was a microphonic pre-amp tube, it took me a minute to find it.
I would suggest not using any 7025 based 12ax7's in any preamp position

 

[frets70]

oooops!

 

[frets70]

sorry for the extra post

 

[Fxrs]

Try using a 7025 based preamp tube! :twisted:

 

[frets70]

why try a 7025? Is that because it is better than a 7024? :lol:

 

[edward]

...I would suggest not using any 7025 based 12ax7's in any preamp position

Are speaking "in your opinion" because of tonal reasons, or are you saying there is "issue" with using one? A 7025 is a 12AT7, and a perfectly acceptable substitution for a circuit designed for a 12AX7. No, it is not an equivalent: it is purportedly a 30% gain reduction, but can handle the same voltage and even higher current as typical 12ax7. One can like them or dislike them in place of an ax7, but this is a decision based on tone, dynamics, and personal preference.

Edward

 

[Fxrs]

why try a 7025? Is that because it is better than a 7024? :lol:

Well...this is one "more", i'n't it? :wink:

 

[frets70]

It is my opinion not to use a 7025, I just tried 3 brand new ones and the went microphonic in a matter of minutes. But the 12AX7 never went microphonic.

Plus every 7025 labeled tube I have ever used seemed to have more high end and not in a good way.

It is all opinion isn't it.

And it is my understanding that a 7025 is not a 12AT7 , the AT7 does have a 30% less gain level, and has the same footprint The AT7 and 7025 are not identical but are the same type and interchangeable.

Quote from RCA manual re 7025:

"This type is identical with miniature type 12AX7 except that it has a controlled equivalent noise and hum characteristic"

All opinion, but I was able to get absolutely killer tones from my DC-3 after experimenting.

 

[edward]

It is my opinion not to use a 7025, I just tried 3 brand new ones and the went microphonic in a matter of minutes. But the 12AX7 never went microphonic.

Plus every 7025 labeled tube I have ever used seemed to have more high end and not in a good way.

It is all opinion isn't it.

And it is my understanding that a 7025 is not a 12AT7 , the AT7 does have a 30% less gain level, and has the same footprint The AT7 and 7025 are not identical but are the same type and interchangeable.

Quote from RCA manual re 7025:

"This type is identical with miniature type 12AX7 except that it has a controlled equivalent noise and hum characteristic"

All opinion, but I was able to get absolutely killer tones from my DC-3 after experimenting.

No biggie; just wanted to clarify the point as your initial post "seemed" to have a definitive air about that statement. But as to tone, clearly it is in the ear of the beholder.

FWIW, the RCA 7025, of which Fender was using all the way back in the heyday of their tweed amps up through the SF era, is interchangeable with the 12AT7 ...even Fender has stated them to be interchangeable.

And the failure you experienced can also be attributable to the tube source, vendor, current-production poor tubes, or crap luck. Moreover, 12AT7s are cabable of handling equal voltage as their ax7 cousins, but can handle even great current draw (which is one reason Fender et.al. recommend one should never put an ax7 into a socket designed for an at7 tube ...you'll kill the ax7.

So yeah, tone/feel should be one's deciding factor. But more than I have found that a good NOS at7 in the place of an ax7 can really sweeten up the tone ...experimentation is key. But as always, YMMV

Edward[/b]

 

[frets70]

Any thoughts on the above issue Fxrs?

What's causing the squealing noise when the amp mentioned above is turned up past 6?

 

[trem]

You guys are way off base here. 7025 is NOT 12AT7. 7025 is a lower noise version of 12AX7, hence the impedance, gain and other characteristics are the same. Perhaps someone here is confusing one with 5751 (which DOES have 30% less gain than a 12AX7)?

 

[frets70]

I can see that everybody has their own idea about tubes but we still haven't assisted the original post.

I still think it is a microphonic tube. Only because my DC-3 had the exact same symptoms.

 

[edward]

Agree with V1 being microphonic. The symptoms are pretty typical of a bad V1, the position that is most sensitive to a bad or marginal preamp tube. Sorry about the diversion OT

Edward

 

[twohailmarys]

update:

i tried changing the preamp tubes again but it still is making the noise.. but i just realized even when i am on the clean channel if the gain knob on the lead is above 6 it makes the noise..so would that mean the gain pot on the lead channelis bad?

 

[Alivefor5]

Twohailmary's, where are the channel masters set?

Where is the output master set?

 

[edward]

Try this:

1. Output master at 3-4.
2. Indiv channel masters for Clean and Lead at 3-4.
3. No cable plugged in
4. Toggle on front plate to Lead channel.
5. Set that first gain knob to 0, and turn up until you get the squeal.
6. Note setting at which you get the squeal.

Now replace the V1 preamp tube (remember, that's the one that is on the input-jack side of the amp) with a known good tube ...or simply one of the others if you don't have spares. Do the above and see if you still get the squeal, and at what setting. Repeat with another tube if need be ...heck, you have 5 others to swap in a DC5.

The point of all this is that the V1 is the amp's first gain stage, and the signal begins here, then splits to the the Clean channel gain stages and the Lead channel gain stages (different tubes or "sides" of tubes, as it were). But the V1 position is the most sensitive to tubes that are bad, marginal, or just downright inhospitable to this spot. So what may be a perfectly well-behaved tube in V2-V6 may make horrid noise (microphonics) in the V1 position. Hope that makes sense ...let us know what ya find

FYI and HUGE apology to all here:
What an idiot!! Yeah, that would be me. A 7025 is most certainly a 12ax7. I don't know why, but it suddenly dawned on me driving on the fwy today, in traffic, listening to the radio, then BAM (in the head, not a crash) ...what the *&%!!!$ was I even thinking; that's a **** elementary point that I screwed up! So very sorry for the confusion, especially in my adamantly insisting it was an at7. What a maroon ...massive brain fart and senile moment. Sorry for the mis-info, gents!!!

Edward

 

[twohailmarys]

after trying every single preamp tube i have it still doesn't change... but i have found that a resistor is burnt on the board:

http://i104.photobucket.com/albums/m163/joevirgona/EBAY026.jpg

does anybody know what value or number it is and do you think this is the problem?

 

[twohailmarys]

i found a schematic for the dc-5 but i don't know how to find the resistor or if it is even on there but maybe someone can..

http://www.schematicheaven.com/boogieamps/boogie_dual_caliber_dc5.pdf

thanks

 

[edward]

i found a schematic for the dc-5 but i don't know how to find the resistor or if it is even on there but maybe someone can..

http://www.schematicheaven.com/boogieamps/boogie_dual_caliber_dc5.pdf

thanks

OK, burnt resistor is definitely bad
Perhaps try starting a new thread with a few pics of the resistor and the surrounding components. Someone here could prob ID where in the schem you'd find that badboy.

Edward