Early Break-Up

[GPoint]

Use 12AT7 (ECC81) for the first stage (it will reduce gain for some 30%), and another one 12AT7 for inverter stage (it works better than 12AX7 there, most original vintage Fender amps used it).

 

[Wally]

What is the service record of thus amp? If the electrolytics are original, they are in need of replacement after 47 years. Break up at 2 indicates a sick amp, ime. If an owner did not want a recap and full service, I would suggest that the amp either be sold or not used until it was properly serviced.

 

[khan]

What GPoint said. Try differnt preamp tubes in the #1 and (optionally) #2 position. I did this with my JCM800 and it really seemed to open up the tonality of the amp. Plus, the added headroom before breakup was a sweet thing to use as well. 12AU7, 12AY7, 12AT7. It's a direct fit/swap. Give it a go. Preamp tubes are generally inexpensive, unless you go for all of the propaganda about brands and vintages. Start out cheap and work your way from there. It's worth a read on reliability of brands and such, as that seems to fluctuate over time.

 

[Wally]

What GPoint said. Try differnt preamp tubes in the #1 and (optionally) #2 position. I did this with my JCM800 and it really seemed to open up the tonality of the amp. Plus, the added headroom before breakup was a sweet thing to use as well. 12AU7, 12AY7, 12AT7. It's a direct fit/swap. Give it a go. Preamp tubes are generally inexpensive, unless you go for all of the propaganda about brands and vintages. Start out cheap and work your way from there. It's worth a read on reliability of brands and such, as that seems to fluctuate over time.

the 5F6A Bassman that is the basis for that Marshall used a 12AY7 in V1. That is the key with those amps, imho. V2 contains the cathode follower, and there are some modern tubes that do not work well there due to the need for a large heater to cathode voltage difference.

 

[elpico]

What GPoint said. Try differnt preamp tubes in the #1 and (optionally) #2 position. I did this with my JCM800 and it really seemed to open up the tonality of the amp. Plus, the added headroom before breakup was a sweet thing to use as well. 12AU7, 12AY7, 12AT7. It's a direct fit/swap. Give it a go. Preamp tubes are generally inexpensive, unless you go for all of the propaganda about brands and vintages. Start out cheap and work your way from there. It's worth a read on reliability of brands and such, as that seems to fluctuate over time.

Two more votes for making the volume knob point at a different number when it breaks up at the same volume it did before. My solution is popular!

I think we could make it even better - put a piece of masking tape over whatever number it's clean to and write "GIGA" on it. Clean to so much number! This amp now has more headroom than any amp ever made.

 

[SanoTele]

1978 was a good year for Deluxes. I vote with the "take it to a tech" advice.

 

[schmee]

My 1978 Delux Reverb breaks up at 2. No clean headroom. Tried different speakers. Has groove tube power tubes.

There is something wrong with the amp. Tell us more about amp, age, service etc. Have filter caps ever been renewed? Could be just a tube. Maybe one power tube has gone bad? Those amps run very high on voltage and some power tubes wont last in it. Get JJ's.
How does it sound in the non reverb channel?

I've played them for 40+ years gigging. Dont mod the amp until you find out what's wrong.

 

[Sparky2]

I dated a gal for a few months, years ago, on Oahu.

We had an early break up, blessedly.

 

[helmiriffs]

Loosen the set screw on the volume knob, make it point at 6 or 7, and retighten the set screw.

LMFAO. Priceless info!

 

[Tim S]

Just curious… does a clean boost pedal with the amp volume BELOW 2 give you the missing clean volume or does it cause the same clipping as if the amp volume is at 2?

 

[Supertwang]

Per the OP, it’s a 1978 DR, and my vote is that it’s probably worth a good amp tech giving it a systematic going over. Several good suggestions here, and it also might need some capacitors replaced. This is a good amp, and it’s worth giving it a bit of expert attention.

That amp is worth $1500-ish working and @ $500-600 not working. Continueing to play it as-is can quickly turn a working amp into a non-working amp. Protect your investment and either fix it or sell it working needing serviced or cut the cord off it and store it.

 

[Chiogtr4x]

1978 was a good year for Deluxes. I vote with the "take it to a tech" advice.

I no longer have my few, vintage Fender amps ($$ was tight/needed to sell...)

but it was my experience ( even if initially having to spend some $$ to get back in shape),
that a good tech & relationship- can keep a vintage tube amp alive and sounding great forever!

 

[Wally]

Put the volume and tone controls in 5. Strum a chord with a n mm efiimnpivk attack. If that is not clean, it is tech time. Or time to sell it to someone who will give the amp the necessary tech attention it deserves and needs.

 

[CB_2005]

I had a 70s and a 65 and neither broke up that early. My guess is filter caps but you need a pro to have a look. It is a necessary investment in a wonderful amp. Good luck!