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Thread: Fender Hotrod Deluxe Tone Problems

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    Fender Hotrod Deluxe Tone Problems

    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

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    Bent Member Chuck H's Avatar
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    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.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Supporting Member Jazz P Bass's Avatar
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    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

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    Quote Originally Posted by Chuck H View Post
    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.

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    Quote Originally Posted by Jazz P Bass View Post
    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
    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

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    Quote Originally Posted by VGuitarist View Post
    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.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by glebert View Post
    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.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Check the power tubesí grid and screen resistors.

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    Quote Originally Posted by xtian View Post
    Check the power tubesí grid and screen resistors.
    Agreed. And as Jazz P Bass said, check the R57 and R58 phase inverter plate resistors on the tube PCB.

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    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.

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    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.

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    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 %.

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    Bent Member Chuck H's Avatar
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    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?

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Supporting Member Randall's Avatar
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    Have we ruled out the speaker?

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    It's weird, because it WAS working fine.....

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    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....

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    To rule out the speaker I would cable over to an external cabinet if one was available.

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    Hack with a Workbench Tone Meister's Avatar
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    If this has been mentioned before then disregard, but is the speaker cable plugged into the correct jack? Viewing from back, the main speaker jack is the one on the left.

    If that isn't connected wrong, does the extension speaker jack have issues? When not used its contacts are normally closed.

    Sorry if this is obvious.

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    Just going to mention this as I've had amps come in with speaker cable plugged into wrong jack.

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    Supporting Member Randall's Avatar
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    In my shop I keep a 5' speaker extension cord that makes quick work of trying different speaker configurations, or connecting to a cab when the chassis is on the bench.
    Click image for larger version. 

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    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    In my shop I keep a 5' speaker extension cord that makes quick work of trying different speaker configurations, or connecting to a cab when the chassis is on the bench.
    Click image for larger version. 

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    Sure, but both sides are RED!!! How will you know if the cable is connected in proper phase

    Just playin' based on some earlier posts.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  21. #21
    Supporting Member Randall's Avatar
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    "Sure, but both sides are RED!!! How will you know if the cable is connected in proper phase"

    I checked it with my meter that is faulty maybe.

    But to your point, one side is ribbed and one side is smooth, but I'm sure you know that.

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    It's weird, because it WAS working fine.....

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    Yes, yes.
    I'm sure most of us have all sorts of adapters a cables made up to test every type of input and output Jack's we may encounter.

    As well as test speakers and loads.

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    Supporting Member Randall's Avatar
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    "Yes, yes.
    I'm sure most of us have all sorts of adapters a cables made up to test every type of input and output Jack's we may encounter."

    I'm guessing the OP doesn't. That is who this was intended for.

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    It's weird, because it WAS working fine.....

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    I did another frequency response test and this time noted levels. The original Fender speaker was definitely degraded - which is something I've never seen before. I have since replaced it with the speaker Fender installs in the Hotrod Deluxe IV - a Celestion Type A, 8 ohm (https://www.amazon.com/dp/B00L6BY5SA...302_E_DDE_dt_1). The amp is fat again and sounds great!

    Once again, thank you all for pushing me in the right direction.

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    Bent Member Chuck H's Avatar
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    Speakers are DEFINITELY the most overlooked component in the tone game. And even at that there are a thousand options out there. I actually goaded tboy to start the "Trade Tent" branch of this forum in the hope that speaker testers/owners could sample more speakers with any given build than their budgets might otherwise allow. Unfortunately that concept hit the ground without even bouncing. But the point is that speakers are important!!! And as an amp builder (designer-ish) I've learned that the right speakers with the right amp can make absolutely all the difference. Short story time ...

    My only local customer also owns a Peavey Classic 50. He didn't want to take his custom "Chuck" amps out for every gig and wanted me to make the Classic 50 more tone-ful and useful for his show. All it took was a speaker change!!! The amp had all the right things going for it response-wise in both channels, except for some subtle final EQ and how the amp responded in the mids to volume for the purposes of acoustic interaction. Since I was just off a prototype situation for the 2009 NAMM show I had been testing a bunch of different speakers for my prototypes. So I had several models on hand. After some trial and error, and REALLY SIGNIFICANT to any Classic 50 owners, it turns out that a pair of Eminence Private Jack's is a freekin' SUPERB match for that amp. The only modification I made to that amp was to replace the original (square magnet, crappy) Peavey speakers with this model and the tone was enviably good by most standards. How the speakers respond and compress (or not) at different frequencies within the useful range is critical. It turns out the Emi PJ's are intended for a closed back cabinet (in general) but in the Classic 50 cab they tone down the ice pick and accentuate the low mids with compression in all the right places. The greatest example of amp/speaker pairing I've experienced to date. So this phenomenon is very real.

    In case no one took notes... You don't need a boutique amp. Or even one of mine! (though I'll stick with it ) Seriously, get a Peavy Classic 50 and load a pair of Private Jacks into it. Chimey Fender cleans and Eric Johnson leads. No $h!t.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Supporting Member Randall's Avatar
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    It takes me about 15 seconds to evaluate a potential speaker problem in my shop. I have a 300 watt 15" cab under my bench with the 1/4" plug within reach. Sorts this type of thing real quick.

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    It's weird, because it WAS working fine.....

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