Fender Hotrod Deluxe Tone Problems

[VGuitarist]

It's me again seeking help for the same Hotrod amp we repaired back in March. This time it is tonal problems...the amp is tinny w/ little low end on both channels.

I'm borrowing a friend's 4th generation Hotrod deluxe amp to help find the root cause. I've been able to prove my pre-amp is fine. This was done by connecting the pre-amp out on my amp to the power amp in on my friend's amp. I then disconnected the physical speaker in my amp to ensure sound only came out of my friend's amp. A simple A/B comparison between the two pre-amps yielded no difference -- they both sounded great. I also reversed the test where my friend's pre-amp out fed into my power amp in and yielded the same results: both pre-amps were the same -- albiet my amp sounds like crap on both pre-amps. So, the issue is in my amp's power amp section. The first thing I did was swapped both 6L6 tubes between amplifiers -- no difference. Two things on the schematic jump out: U1 (TL072) and R57 (presence adjustment). Before touching them, I wanted to post here first. oh, I replaced all 12AX7 and 6L6 tubes back in March.

Any thoughts on what could be causing this? Should I replace the op amp or tweak the presence adjustment? What else should I look at? I can't upload the schematic due to a quota issue.

Thanks in advance for your help.

-VGuitarist

[Chuck H]

The presence circuit is a pretty simple and old school design. Describing the amp as sounding "tinny" doesn't make think of an amp with more presence. Do you remember the amp sounding tinny with the presence control turned up? Now if your feedback resistor were shorted that might make the amp sound weak and thin. But resistors rarely fail short. They usually fail open. If the presence control is working I wouldn't suspect that circuit for now. If it's not and you think presence (in general) sounds tinny then I guess you could suspect the presence circuit. Though as with resistors, pots usually fail open.

The schematic looks to me that U1a before the preamp out (so that's not it) and U1b isn't in the circuit unless the power amp in jack is used. Does your amp sound tinny when the power amp in jack is not used? If it does then U1 isn't the problem either.

But other than those two circuits there isn't much to suspect, as you noticed. I suppose you could try another tube in the phase inverter before cracking it open. If that doesn't help you're down to bench testing for voltages and the maybe the OT.

P.S. Stop disconnecting the speakers to quiet yours and your friends amplifier. The preamp out jack doesn't interrupt the signal so your power amp is still trying to make watts when it's in use. Disconnecting the speaker is disconnecting the load from a working power amp. That is a very bad thing to do to a tube amplifier and may detriment very expensive parts like the output transformer. If you have been doing this to your own amp for recording and such it might explain why you have a problem. If you keep doing it to your friends amp you may both have a problem soon.

[Jazz P Bass]

As far as I can tell U1 is not the issue.
U1A is the preamp out driver.
U1B is the final driver before the power amp in.

Not too sure why you are considering 'modding' the presence control to repair the amplifier.

The first thing that I would check is the phase inverter plate resistors (R57/ R58) for an open reading.
One of them may be open circuit.

Schematic:
Hot_Rod_Deluxe_Schematic.pdf

[VGuitarist]

The presence circuit is a pretty simple and old school design. Describing the amp as sounding "tinny" doesn't make think of an amp with more presence. Do you remember the amp sounding tinny with the presence control turned up? Now if your feedback resistor were shorted that might make the amp sound weak and thin. But resistors rarely fail short. They usually fail open. If the presence control is working I wouldn't suspect that circuit for now. If it's not and you think presence (in general) sounds tinny then I guess you could suspect the presence circuit. Though as with resistors, pots usually fail open. My bad on the presence comment. For some reason, I thought there was another presence potentiometer on the circuit board. Both presence and reverb controls are turned all of the way down on both amps.

The schematic looks to me that U1a before the preamp out (so that's not it) and U1b isn't in the circuit unless the power amp in jack is used. Does your amp sound tinny when the power amp in jack is not used? If it does then U1 isn't the problem either. The amp sounds tinny all of the time (with or without the power amp in connection). I connected the preamp from the other amplifier to determine which section was causing the problem (preamp or power amp).

But other than those two circuits there isn't much to suspect, as you noticed. I suppose you could try another tube in the phase inverter before cracking it open. If that doesn't help you're down to bench testing for voltages and the maybe the OT.

P.S. Stop disconnecting the speakers to quiet yours and your friends amplifier. The preamp out jack doesn't interrupt the signal so your power amp is still trying to make watts when it's in use. Disconnecting the speaker is disconnecting the load from a working power amp. That is a very bad thing to do to a tube amplifier and may detriment very expensive parts like the output transformer. If you have been doing this to your own amp for recording and such it might explain why you have a problem. If you keep doing it to your friends amp you may both have a problem soon.

I get it...the amp needs a load. I only did it a couple of times. It is connected now.

My amp sounds bad using either pre-amp. The other amp sounds good using either pre-amp.

[VGuitarist]

As far as I can tell U1 is not the issue.
U1A is the preamp out driver.
U1B is the final driver before the power amp in.

Not too sure why you are considering 'modding' the presence control to repair the amplifier.

The first thing that I would check is the phase inverter plate resistors (R57/ R58) for an open reading.
One of them may be open circuit.

Schematic:
[ATTACH]54782[/ATTACH]

The presence comment was minor confusion on my part...I thought there was a second presence pot on the circuit board. I will check R57/R58. Thanks

[glebert]

I get it...the amp needs a load. I only did it a couple of times. It is connected now.

My amp sounds bad using either pre-amp. The other amp sounds good using either pre-amp.

FWIW usually plugging anything into the return jack will disconnect it from the preamp, I usually just use a bare plug or an extra patch cord to break the connection.

[Chuck H]

FWIW usually plugging anything into the return jack will disconnect it from the preamp, I usually just use a bare plug or an extra patch cord to break the connection.

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Plugging an unterminated cord into the power amp in jack will prevent signal from getting to the power amp.

[xtian]

Check the power tubes’ grid and screen resistors.

[Tone Meister]

Check the power tubes’ grid and screen resistors.

Agreed. And as Jazz P Bass said, check the R57 and R58 phase inverter plate resistors on the tube PCB.

[VGuitarist]

As part of a different thread (capacitors buzzing in the power supply), I replaced R57 and R58 with 2 watt resistors. They measure fine. I have attached a schematic showing all of the mods. Here is an excerpt from the thread:


The noise issue is fixed -- the snubber capacitors C62, C63, C64 and C65 were definitely oscillating. I replaced them with 8200pf @ 1.6KV metal film capacitors. In addition to the capacitors, I replaced the rectifier diodes D6, D7, D8 and D9 (1N4006) with UF4006 fast switching diodes. The circuit board is ultra quiet now.

While I had the amp open, I followed your recommendation and replaced a handful of other components.

1. Purchased a capacitor replacement kit from Fromel Electronics. It covered the larger capacitors C31 (47uf/500v), C33 (22uf/500v), C35 (22uf/500v) and C36 (22uf/500v) and a few smaller capacitors: C1 (47uf/25v), C8 (1uf/100v), C9 (22uf/35v) and C56 (47uf/25v). To be honest, a few of their caps were under-rated, so I backfilled them with parts from Mouser electronics (C1, C9 and C56).
2. Replaced R61 and R62 screen resistors (470 ohm) with 2-watt metal film (flameproof) resistors.
3. Replaced plate resistors R11 (100K), R4 (100K), R16 (100K), R22 (100K), R57 (82K) and R58 (100K) with 2-watt metal film (flameproof) resistors.
4. I ordered a tube kit from Tube Depot and replaced all 5 tubes (3x Tung-Sol 12AX7 and 2x Tung-Sol 6L6GCSTR)

Beyond that, I own a nice Pace solder/de-soldering station, so I de-soldered/re-soldered every connection. I also lifted the two 5-watt resistors (R78 and R79) off the board. This job required me to purchase neutral cure silicone (Amazon) to glue the large filter caps to the circuit board. All of the component parts came from Mouser Electronics. If you want part numbers, please let me know.

[Mick Bailey]

A thin sound can be caused by a defective coupling cap where you get just a few pf in parallel with board capacitance. Make sure you have a strong signal either side of C24.

[VGuitarist]

No breakthrough yet on finding the root cause. Interestingly, I did a frequency response sweep of the amp (20 Hz - 20 kHz) and it is perfect (generator on the input, measured at TP23 and on the speaker lead). Further, I also watched the frequency sweep with an oscilloscope. Every tone yielded a perfect sine wave. This is puzzling, since the amp is noticeably thinner than my friend’s Hotrod - especially when I add distortion/overdrive. It almost sounds like the presence or reverb is turned up too high — even though I have both knobs all of the way down. Is it possible one of the OPAmps (U1-B or U2-A) degraded and is causing the thin sound?

I’m in the process of comparing components between the amps. So far, everything matches within a few %.

[Chuck H]

That amp is voiced like most guitar amps. That being the case I wouldn't expect a flat frequency response from 20 to 20k. Rather, I would expect a considerable roll off in amplitude at either end of the spectrum. Can you define your test parameters? What was your input signal in mV? What sort of load are you using? What is the amps performance in watts?

Considering your friends amp... Do both amps have the same speaker? Produce the same volume before clipping? Are these comparisons being done with the same guitar?

[Randall]

Have we ruled out the speaker?

[1ampman]

You need to remove the speaker from the cab to see if is frozen. That's the only way to tell. Gently push on the cone to see if it's free to vibrate. I had an amp with the same description. The dude burnt our a Vintage 30.....No bottom/bass is describes the sound, Tinny is another way....